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
Please, have a look at this commit:
https://github.com/cardinot/mixxx/commit/4ed48b1ad08bf66014a27b53d579d2e3c99105a1

I managed to find out how it works and how Mixxx update the columns... I don't know if there's another way to do it without using TIO again, but if it's the only way, I would go for "don't do that"...