Add support of TAG_MUSICBRAINZ_RELEASEGROUPID

Closes #1767

src/db/plugins/ProxyDatabasePlugin.cxx

@@ -182,6 +182,8 @@ static constexpr struct {
 #endif
 #if LIBMPDCLIENT_CHECK_VERSION(2,21,0)
 	{ TAG_MOOD, MPD_TAG_MOOD },
+	{ TAG_MUSICBRAINZ_RELEASEGROUPID,
+	  MPD_TAG_MUSICBRAINZ_RELEASEGROUPID },
 #endif
 	{ TAG_NUM_OF_ITEM_TYPES, MPD_TAG_COUNT }
 };

src/tag/Id3MusicBrainz.cxx

@@ -15,5 +15,7 @@ const struct tag_table musicbrainz_txxx_tags[] = {
 	{ "MusicBrainz Release Track Id",
 	  TAG_MUSICBRAINZ_RELEASETRACKID },
 	{ "MusicBrainz Work Id", TAG_MUSICBRAINZ_WORKID },
+	{ "MusicBrainz Release Group Id",
+	  TAG_MUSICBRAINZ_RELEASEGROUPID },
 	{ nullptr, TAG_NUM_OF_ITEM_TYPES }
 };

src/tag/Names.cxx

@@ -42,6 +42,7 @@ static constexpr struct tag_table tag_item_names_init[] = {
 	{"MUSICBRAINZ_TRACKID", TAG_MUSICBRAINZ_TRACKID},
 	{"MUSICBRAINZ_RELEASETRACKID", TAG_MUSICBRAINZ_RELEASETRACKID},
 	{"MUSICBRAINZ_WORKID", TAG_MUSICBRAINZ_WORKID},
+	{"MUSICBRAINZ_RELEASEGROUPID", TAG_MUSICBRAINZ_RELEASEGROUPID},
 };
 
 /**

src/tag/Type.hxx

@@ -43,6 +43,7 @@ enum TagType : uint8_t {
 	TAG_MUSICBRAINZ_TRACKID,
 	TAG_MUSICBRAINZ_RELEASETRACKID,
 	TAG_MUSICBRAINZ_WORKID,
+	TAG_MUSICBRAINZ_RELEASEGROUPID,
 
 	TAG_NUM_OF_ITEM_TYPES
 };