New gallery plugin

These commits add a new gallery plugin to MythTV. 
The main differences to the current MythGallery plugin are:
- The program logic is on the backend and all functions are accessible via the service API
- The presentation logic is directly incorporated in the frontend like MythVideo.
- It uses the backend database to save the directory and file information together with the exif image orientation
- Storage groups are supported

General information:
- The images are stored on the backend server in a storage group. The images will then be synced into the database on the backend. Multiple frontends can therefore connect to the backend and view the images. This is similar to the MythVideo plugin.
- The exif information is also read from each file during sync and stored in the database. If no exif value is available the user can still rotate the image and it will be remembered because this information is available in the database.
- The synchronization progress is completely done in the backend and the status is available via the service API. It can be started or stopped via the service API or the theme.
- The sync runs in the background and navigating the folder structure and all file operations are therefore possible during sync.
- Thumbnails will be generated from images and also from videos.
- Each directory thumbnail image will show up to four image previews like XBMC or KDE does.
- The images are shown either one by one or as a slideshow which is configurable
- Videos will be played by the internal player.
- The user can rotate, flip, delete and rename files.
- Hiding and unhiding files is also possible in case these shall not be visible to the audience. This information is saved in the database.