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
> @cardinot Can you do this first in a new branch and let Daniel check if this really solves his issues.

Of course. I also pushed a commit that makes the widget ignores the rescaling function...
https://github.com/cardinot/mixxx/tree/coverArtSupport_pixmap

@daschuer, could you test it please?