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
What about reading the pixmap directly from qpixmapcache?
success case:
1 - requestPixmap(trackId, coverLoc)
2 - emit(pixmapCached(trackId, coverLoc))
3 - slotPixmapCached(trackId, coverLoc)
4 - QPixmapCache::find(coverLoc, &pixmap)