PICARD-418: fallback to using release group cover

It adds the option to use release group cover art from CAA.
It is required to enable CAA and release group fallback is disabled by default

http://tickets.musicbrainz.org/browse/PICARD-418
http://tickets.musicbrainz.org/browse/PICARD-53
I'm not sure why there are separate classes for release group images without differences in the behaviour?