Cover art support

- implement database scheme
- search for embedded cover
- search for cover in track directory
- save covers in disk cache
- search for covers in disk cache
- minimal ui (display covers in lower left corner)
- handle existing users
- LRU cache
doing the actual search for the cover in the track directory and id3tags can be done in a separate thread with QtConcurrent, this actually not a bad idea. But db queries should be executed only in the main thread.

The problem is that you might be blocking the call of another mixxx function that changes the library (adding a song to a playlist) which then will fail silently.