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
- [x] unit tests
I realized that as our problem occurs just when the user is holding down an arrow key... we just would have to check if it is happening, and we could use a simple keyevent for it... 5c8d029fcc8d37a8f80c9e9e42550cd38f35e5a0