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. 

It does when you skip the playing tracks

>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 arrow key.

in what window to you look at the cover. For me banshee only displays the cover of the currently playing song as clementine.

> Why not "requestPixmap(trackId, coverLoc, &m_pixmap)" ?

@cardinot Can you do this first in a new branch and let Daniel check if this really solves his issues.