Cover art support

- [x] implement database scheme
- [x] search for embedded cover
- [x] search for cover in track directory
- [x] save covers in disk cache
- [x] search for covers in disk cache
- [x] minimal ui (display covers in lower left corner)
- [x] handle existing users
- [x] LRU cache
- [ ] unit tests
yeah sounds good. Banshee has a plugin to copy the downloaded covers back to the album folders, so they could come from banshee although they are not the ones used by banshee