show fullsize cover in all of it's glory
Add method to ask Cache if we know the track

Sometimes we rely on that the covers are saved in the database. In the
few widgets that do this we can now check with this method if it is
already in the cache and about the be stored in the DB soon.
Merge branch 'fullsize-cover' of https://github.com/kain88-de/mixxx into coverArtSupport
fixing CoverArtCache::trackInDBHash - it must return the coverLocation instead of the md5hash