BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an input apparatus for controlling a plurality of electronic devices. 2. Description of the Related Art Coming rapidly into widespread use today are two-way cable systems and computer-based commercial networks including what is known as the Internet utilizing electronic mail (E-mail) and the World Wide Web (WWW). Such networks are no longer limited in their use to computers on the desk top; they are fast finding their way into various electronic devices that make up an audiovisual (AV) system in the household living room. For example, terminal equipment connected to the Internet or like networks for computer communication through telephone lines may be hooked up with a television monitor device. With the terminal connected illustratively to the Internet, the monitor may display text information, graphics and icons. Control of and text input to such terminal equipment are generally effected by use of a keyboard connected to the terminal via a cable. Some keyboards developed in recent years are capable of transmitting to the connected terminal information generated by key operations in wireless fashion using infrared signals (IR (infrared ray) keyboards). At present, AV devices are controlled by their respective controllers. If keyboard-controlled terminal equipment of the above-described type is connected to a plurality of AV devices each controlled by a dedicated controller, the user is required to manipulate both the multiple controllers and the keyboard. The operations involved in such cases are complicated and difficult to accomplish. Many of today's AV devices are equipped with a graphic interface that allows graphics to be displayed illustratively on a connected monitor. The trouble with such setups is that to display graphics, each AV device must be operated with its dedicated controller. SUMMARY OF THE INVENTION It is therefore an object of the present invention to overcome the above and other disadvantages of the related art and to improve the related art. In carrying out the invention and according to one aspect thereof, there is provided an input apparatus comprising: a first operation key part having a plurality of operation keys; a second operation key part having a plurality of operation keys; a control key part having at least one control key; control means for generating operation information for a first electronic device when any operation key of the first operation key part is operated, the control means further generating operation information for the first electronic device when, with no control key of the control key part operated, any operation key of the second operation key part is operated, the control means further generating operation information for a second electronic device when, with any control key of the control key part operated, any operation key of the second operation key part is operated; and transmission means for transmitting the operation information generated by the control means. Preferably, the input apparatus may further comprise a third operation key part having at least one operation key, wherein the control means generates operation information for the second electronic device when any operation key of the third operation key part is operated regardless of whether any control key of the control key part is operated. In the above preferred structure, the transmission means may preferably transmit the operation information as an infrared signal. According to another aspect of the invention, there is provided an input apparatus comprising: a first operation key part having a plurality of operation keys; a second operation key part having a plurality of operation keys furnished separately from the first operation key part; control means for generating operation information for a first electronic device when any operation key of the first operation key part is operated, the control means further generating operation information for a second electronic device when any operation key of the second operation key part is operated; and transmission means for transmitting the operation information generated by the control means. Preferably, the transmission means in the above input apparatus may transmit the operation information as an infrared signal. According to a further aspect of the invention, there is provided an input apparatus comprising: a first operation key part having a plurality of operation keys; a second operation key part having a plurality of operation keys furnished separately from the first operation key part; cursor operation keys for moving a cursor; control means for generating operation information for a first electronic device when any operation key of the first operation key part is operated, the control means further generating operation information for a second electronic device when any operation key of the second operation key part is operated, the control means further generating operation information for the first and the second electronic devices when any of the cursor operation keys is operated; and transmission means for transmitting the operation information generated by the control means. Preferably, the control means in the above input apparatus may generate operation information for the first electronic device and operation information for the second electronic device alternately on a time division basis when any of the cursor operation keys is operated. With this structure, the transmission means may preferably transmit the operation information as an infrared signal. According to an even further aspect of the invention, there is provided an input apparatus comprising: a base; a first operation key part having a plurality of operation keys formed on the base; a cover part attached movably to the base, the cover part covering the first operation key part when set to a first position, the cover part exposing the first operation part and located under the base when set to a second position; a second operation part having a plurality of operation keys and attached to the cover part so as to be exposed when the cover part is set to the first position; control means for generating operation information for a first electronic device when any operation key of the first operation key part is operated, the control means further generating operation information for a second electronic device when any operation key of the second operation key part is operated; and transmission means for transmitting the operation information generated by the control means. Preferably, the input apparatus may further comprise position determination means for determining whether the cover part is set to the first position or to the second position; wherein, if the position determination means has determined that the cover part is set to the first position, the control means does not generate operation information in response to any operation key of the first operation key part being operated and only generates operation information in response to any operation key of the second operation key part being operated, and wherein, if the position determination means has determined that the cover part is set to the second position, the control means does not generate operation information in response to any operation key of the second operation key part being operated and only generates operation information in response to any operation key of the first operation key part being operated. Preferably, the transmission means in the above input apparatus may be attached to one edge of the base, with the cover part being mounted pivotably on another edge of the base. With this structure, the transmission means may preferably transmit the operation information as an infrared signal. The inventive input apparatus has the first operation key part for controlling the first electronic device, the second and third operation key parts for controlling the second electronic device, and/or the cursor operation keys for controlling the first and second electronic devices. The control means generates operation information for the first and second electronic devices and transmits the generated information to the devices. The present invention relates to an input apparatus for controlling a plurality of electronic devices comprising: • a base;