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
I did my test on a single core atom netbook with a SSD. I am just measuring the time scrolling trough the list using the arrow keys.
On a second thought I think my bullet-list above is not, my case, because I can do the test many times on the same Mixxx instance without any difference. So lets assume a settled cache. 
In this case every cover is drawn in the call tree of selectionChanged() because no threads are involved. Right?  
But this is odd, because the covers are not shown if you rush over the list. it just slows down the scrolling. 
A timer would help here. This can be re triggered with 50 ms and fires only if a selection stays on a Item for 50 ms.