musicbrainz-coverart: Add support for release group coverart
In Musicbrainz terminology, a Release is "the unique release (i.e. issuing) of a product on a specific date with specific release information such as the country, label, barcode and packaging" while a Release Group "is used to group several different releases into a single logical entity."
The musicbrainz plugin is only able to build a thumbnail with a release id. However, most of the time, a release does not have a coverart. In that case, the returned thumbnail should be the one from the ("release-group")[https://musicbrainz.org/doc/Cover_Art_Archive/API#.2Frelease-group.2F.7Bmbid.7D.2Ffront.5B-.28250.7C500.7C1200.29.5D] if it exists.
It needs the following changes:
- in acoustid plugin, retrieve the release-group id and store it in a runtime key
- in musicbrainz-coverart, add the following logic:
- try to get the coverart from the release id
- if the release coverart does not exist, try to get the one associated with the relase-group id