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
folder.jpg is by definition of windows the folder icon. This will be in most cases the same than the cover art, but since Mixxx should display the cover art, it should prefer album.jpg over foder.jpg