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
I suspect the use of QPixmap is already an indirect `assert` because you can't create them outside of the UI-thread. I haven't tested it lately but if I remember right calling this from a non UI-Thread will result in a segfault