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
Did you improve anything? 
The word scrolling in may was misleading, I should write changing focus via keyboard. 

I cant see the speed dependency any more, It is still much slower than the original, but usable.

But I still think we need a delay because of this use case:
Move focus from one track via keyboard to a track 20 rows below and than back to the original track. 

Currently this happens: 
* 20 future workers are instantiated
* they are processed by all threads available in the (I think global) tread pool for future workers. 
* Rest is queued 
* One thread gets HDD others are blocked at probably blocked 
* If If all workers are ready we copy 20 Pixamp (19 useless) through Mixxx
* Pixmap cache is filled with 20 Pixamp. 
* The original might get flushed. So if we move back, we have no benefit fro our cache.

With a short timer slot, we will load only a cover if it is required and most drawbacks from the list above are gone.