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
yeah we do. e.g.

https://github.com/mixxxdj/mixxx/blob/master/src/library/dao/trackdao.cpp#L112

you can see the other uses by grepping for 'return -1'