style fix
Add test for CoverArtCache::changeImage

Good damn it this one keeps failing with a segfault. When I try to
set the CoverArtDAO for the cache it happens. I have no clue why.
FIX change cover art to work with embedded covers

Change cover art now works with embedded covers. Plus now it is possible
to decide between unsetting a cover and setting it to an embedded one.
Merge branch 'fix-reload-embedded-cover' of https://github.com/kain88-de/mixxx into coverArtSupport
fixing CoverArtCacheTEST - segfault when destroying