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
> To avoid designing feature creep functions always check if you function name contains everything the function does and still fits into 80 chars. If it is too long you should split up the function

Nice rule :-) we should add it to our code style guide.