Add grilo-tracker mapping for MusicBrainz Identifiers
Tracker stores MusicBrainz ids now. (tracker!91 (merged))
Grilo Media identifies MusicBrainz Track ID, Artist ID, Recording ID.
Add custom keys Release ID and Release Group ID since they're not yet identified by grilo.
Add a mapping between tracker and grilo so that we can query MusicBrainz ids in Music.
Edited by Sumaid