Cover art support

- implement database scheme
- search for embedded cover
- search for cover in track directory
- save covers in disk cache
- search for covers in disk cache
- minimal ui (display covers in lower left corner)
- handle existing users
- LRU cache
this window blocks the rest of mixxx for me. I'm not 100% convinced that this is large cover is of value to a DJ but it shouldn't block the other controls of mixxx