MusicBrainz support for CD plugin
MusicBrainz (MB) is a music database that contains, most of the time, better quality tracklists for CD, than CDDB/FreeDB/GnuDB.
Querying MB would require computing a disc ID with https://musicbrainz.org/doc/libdiscid
GnuDB.org could be kept as fallback.
MB support could also be extended to non-CD, we could for example lookup the release we want to tag with pasting its MB URL: https://musicbrainz.org/release/eaa6fe6f-bb5b-4c57-ba94-181d6c9c7c73