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
What about move CoverArtDAO::updateLibrary to TrackDAO ?
It means, also having the TrackDAO instance into CoverCache to update the Library table... instead of doing it into CoverDAO...?