Cover art support

- [x] implement database scheme
- [x] search for embedded cover
- [x] search for cover in track directory
- [x] save covers in disk cache
- [x] search for covers in disk cache
- [x] minimal ui (display covers in lower left corner)
- [x] handle existing users
- [x] LRU cache
- [x] unit tests
Clementine does not change the covers when scrolling though a play list. 
Banshee seems to have a timer solution because they do not change the covers when you scrolling though the artist list, until you release the error key. 

@cardinot 
Yes. your idea will save us from one Pixamp copy. 
The only drawback is that we loose the wrapper class nature if we use QPixmapCache from everywhere. 
Why not "requestPixmap(trackId, coverLoc, &m_pixmap)" ?