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
So, to summarize... let's use **coverArtSupport_pixmap** + **your patch** + delay in **WTrackTableView::selectionChanged** ?

About the delay... Do we have something like that implemented somewhere on Mixxx?