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
>conclusion:  as only 6 tracks have cover, he expect to see just them
with a cover

Yeah, I agree! Thanks @daschuer 

I also realized that using this approach we would be loading the same Pixmap in our qpixmapcache most of time (because in general tracks of the same album will have the same picture)...
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...