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
Why do you need that here? Under which circumstances would this code branch be executed? I suspect you fear that the tables in the tablemodels don't get updated while mixxx runs. Did you check BaseTrackCache, you can tell it that some tracks changes and reload the information for these tracks into the tablemodels.