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
Not that I know of if you want to work asynchronously in Qt. We could send a reference.
From where do you know that sending pixmaps does a lot of copying and mallocs?