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
Hm I don't like that. can't you just update the picture? In the other extra windows that we have we update the information inside of the window and keep the rest of mixxx responsive