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
>YES! We need a delay. Scrolling is significant slower factor ~5 but that is not the main problem because the original scrolling was really fast. The biggest problem is that the speed depends on if there is a cover and probably the cover size as well. That feels odd.

We still need a delay?