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
> I missed that commit where you stopped using the disk-cache
No problem! We stop doing it after this commit: e91328cb1689c5c55f9f9750de4123d8f5fb7d88