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
The parent class we use for library models is pretty stupid. This signal is needed so that we can inform wcoverart that a different track is being selected. 'focusedTrackChanged' is more descriptive so we could change it