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
Nice solution, but this did not work for Midi mappings :-( Not sure if this is a real issue though. but there are scripts that are sending repetitive events https://github.com/mixxxdj/mixxx/blob/831a1450a70ea5d19b6a44fed6b6eb71d8ba9d0b/res/controllers/Hercules-DJ-Console-Mk4-scripts.js#L59