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
The disk cache for all covers has also the drawback that it behavior might not be expected. 
If a folder contains a "cover.jpg" it is copied to the cache if it found the first time. 
If the user exchanges the cover, Mixxx is still displaying the cover in the cache.  

IMHO a <track-file-base>.jpp in the track folder should be have a high priority this gives the user full control.

What about this 
1. Track cache (is only used if the user configures a different cover in Mixxx) 
2. ./<track-file-base>.jpp
3. embedded
4. cover.jpg 
6. anything else found in the folder 

It could be also interesting to specify a cover cache from an external player as cover source.