DISPLAY DEVICE, DISPLAY METHOD, AND PROGRAM CROSS-REFERENCE TO RELATED APPLICATIONS [001] The present application claims priority to and contains subject matter related to that disclosed in Japanese Priority Patent Application JP 2010-107358 filed in the Japan Patent Office on May 7, 2010, the entire content of which is hereby incorporated by reference. BACKGROUND Technical Field [002] The present disclosure relates to display devices, display methods, and programs for displaying thumbnails. Description of the Related Art [003] In recent years, there is greater number of digital broadcasting programs due to greater number of channels, and the moving image contents on the network are also exploding and a great number of moving images can now be easily reproduced. The content of the moving image content is generally expressed with one thumbnail. The user can thus speculate the content of the content to a certain extent before reproducing the content by looking at the thumbnail image. [004] However, the content of the moving image content may not be easily speculated by simply looking at the thumbnail when it is difficult to extract the appropriate thumbnail from the moving image content or when it is difficult to display all the moving image contents with the extracted plurality of appropriate thumbnails. As a result, troublesome effort and great amount of time are wasted to look for the desired scene in the desired moving image from the moving image content. Therefore, a means for comfortably browsing through the appropriate number of thumbnails showing the flowing of content of the content is desired to instantaneously determine the content of the moving image content. [005] Various techniques have been proposed from the related art as a means for extracting an appropriate thumbnail from the moving image content and enabling the desired scene to be easily searched. For instance, Japanese Patent Application Laid-Open No. 2002-27400 describes extracting a representative image in which a desired program or scene can be selected. In Japanese Patent Application Laid-Open No. 2002-27400, the still images are extracted using a video signal and are displayed in a list, where a method of easily searching for the moving image content is provided to the user. [006] Japanese Patent Application Laid-Open No. 2009-246722 relates to a method of extracting a thumbnail of a certain scene from the moving image content, where the thumbnail is extracted and displayed based on a plurality of rules that differ for every content so that the content of the content can be speculated to a certain extent by simply looking at the thumbnail image before reproducing the content. TECHNICAL PROBLEM [007] However, Japanese Patent Application Laid-Open No. 2002-27400 is a technique for extracting only one representative image as a thumbnail, and does not provide a method for displaying the list of thumbnails representing the flow of content of the content in view of the importance of the scene, and the like. Thus, the content of the video is difficult to instantaneously and accurately grasp by the display of thumbnails in Japanese Patent Application Laid-Open No. 2002-27400. [008] Japanese Patent Application Laid-Open No. 2009-246722 relates to the technique of "extracting thumbnail" to accurately determine the content of the video. Thus, Japanese Patent Application Laid-Open No. 2009-246722 does not provide a method of displaying a list of thumbnails while changing the display method of the thumbnail showing an important scene from the display method of other thumbnails. [009] In light of the foregoing, it is desirable to provide a novel and improved display device, display method, and program capable of changing the display method of the thumbnail according to the degree of importance. SUMMARY [010] Accordingly, there is provided an apparatus for displaying thumbnail images corresponding to portions of content. The apparatus comprises an identifying unit for identifying a thumbnail image representing a portion of content, and an importance level of the portion of content represented by the thumbnail image. The apparatus further comprises a selection unit for selecting a display mode of the thumbnail image according to the identified importance level. The apparatus further comprises a generation unit for generating information controlling display of the thumbnail image according to the selected display mode. [011] In another aspect, there is provided a method for displaying thumbnail images corresponding to portions of content. The method comprises identifying a thumbnail image representing a portion of content. The method further comprises identifying an importance level of the content represented by the thumbnail image. The method further comprises selecting a display mode of the thumbnail image according to the determined importance level. The method further comprises displaying the thumbnail image according to the selected display mode. [012] In another aspect, there is provided a tangibly embodied non-transitory computer-readable storage medium storing instructions which, when executed by a processor, cause a computer to perform a method for displaying thumbnail images corresponding to portions of content. The method comprises identifying a thumbnail image representing a portion of content. The method further comprises identifying an importance level of the content represented by the thumbnail image. The method further comprises selecting a display mode of the thumbnail image according to the determined importance level. The method further comprises displaying the thumbnail image according to the selected display mode. [013] According to the embodiments described above, the display method of the thumbnail can be changed according to the degree of importance of the thumbnail. BRIEF DESCRIPTION OF THE DRAWINGS [014] FIG. 1 is an overall configuration diagram of a terminal according to first and second embodiments; [015] FIG. 2 is a view showing a variant of the overall configuration; [016] FIG. 3 is a view showing one example of a content storage unit according to a first embodiment; [017] FIG. 4 is a view showing a display example of the thumbnails according to the first embodiment; [018] FIG. 5 is a flowchart showing the display method of the thumbnails according to the first embodiment; [019] FIG. 6 is a flowchart showing the display method of the thumbnails according to the first embodiment; [020] FIG. 7 is a flowchart showing the display method of the thumbnails according to the first embodiment; [021] FIG. 8 is a view showing a display example of the thumbnails according to the first embodiment; [022] FIG. 9 is a view showing a display example of the thumbnails according to the first embodiment; [023] FIG. 10 is a view showing a display example of the thumbnails according to the first embodiment; [024] FIG. 11 is a view showing a display example of the thumbnails according to the first embodiment; [025] FIG. 12 is a view showing a display example of the thumbnails according to the first embodiment; [026] FIG. 13 is a view showing one example of a content storage unit according to a second embodiment; and [027] FIG. 14 is a view showing a display example of the thumbnails according to the second embodiment. DETAILED DESCRIPTION OF THE EMBODIMENTS [028] Hereinafter, exemplary embodiments will be described in detail with reference to the appended drawings. Note that, in this specification and the appended drawings, structural elements that have substantially the same function and structure are denoted with the same reference numerals, and repeated explanation of these structural elements is omitted. [029] Embodiments will be described in the following order. 1.