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
> when did this method stop to also delete the files from disk?

I'm not sure if I got your point, but we are not using the disk-cache anymore, it means that we do not have files to remove...