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 don't know if it's clear enough... but imagine that the 6 covers are
> exactly the same picture and each of them has 1MB... it means that we
> would be spending 5MB of our cache...
> I don't know if QPixmapCache hash the pixmaps internally to avoid it...
>

Yes, this is defiantly an issue, imagine a 30 Tracks 2 CD Sampler. This
could be solved by storing and comparing a MD5 checksum.
But since it works fine without, you should talk to Max if this is part
of your project.