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
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.