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
When a new cover is loaded the widget will reset back to the default until we get the signal that the cover was found. Even if we would listen to the `pixmapNotFound` signal there would be nothing extra we could do with that information