Support MusicBrainz release track ID

I still don't totally understand when MusicBrainz uses a track ID and
when it uses a release track ID - they're both displayed as the
"MusicBrainz Track ID" tag in Picard, despite being treated as different
tags by everything else - but supporting both is easy enough.

src/mpd_now_playable/song.py

@@ -19,6 +19,7 @@ class Song:
 	queue_length: int
 	file: Path
 	musicbrainz_trackid: UUID | None
+	musicbrainz_releasetrackid: UUID | None
 	title: str | None
 	artist: str | None
 	composer: str | None