move MusicBrainz id3v2 tags to separate file

We will reuse these tags elsewhere.

Makefile.am

@@ -923,6 +923,7 @@ libtag_a_SOURCES =\
 	src/tag/ReplayGain.cxx src/tag/ReplayGain.hxx \
 	src/tag/MixRamp.cxx src/tag/MixRamp.hxx \
 	src/tag/Generic.cxx src/tag/Generic.hxx \
+	src/tag/Id3MusicBrainz.cxx src/tag/Id3MusicBrainz.hxx \
 	src/tag/ApeLoader.cxx src/tag/ApeLoader.hxx \
 	src/tag/ApeReplayGain.cxx src/tag/ApeReplayGain.hxx \
 	src/tag/ApeTag.cxx src/tag/ApeTag.hxx

src/tag/Id3MusicBrainz.cxx

@@ -0,0 +1,34 @@
+/*
+ * Copyright 2003-2017 The Music Player Daemon Project
+ * http://www.musicpd.org
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#include "Id3MusicBrainz.hxx"
+#include "TagTable.hxx"
+#include "TagType.h"
+
+const struct tag_table musicbrainz_txxx_tags[] = {
+	{ "ALBUMARTISTSORT", TAG_ALBUM_ARTIST_SORT },
+	{ "MusicBrainz Artist Id", TAG_MUSICBRAINZ_ARTISTID },
+	{ "MusicBrainz Album Id", TAG_MUSICBRAINZ_ALBUMID },
+	{ "MusicBrainz Album Artist Id",
+	  TAG_MUSICBRAINZ_ALBUMARTISTID },
+	{ "MusicBrainz Track Id", TAG_MUSICBRAINZ_TRACKID },
+	{ "MusicBrainz Release Track Id",
+	  TAG_MUSICBRAINZ_RELEASETRACKID },
+	{ nullptr, TAG_NUM_OF_ITEM_TYPES }
+};

src/tag/Id3MusicBrainz.hxx

@@ -0,0 +1,25 @@
+/*
+ * Copyright 2003-2017 The Music Player Daemon Project
+ * http://www.musicpd.org
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#ifndef MPD_TAG_ID3MUSICBRAINZ_HXX
+#define MPD_TAG_ID3MUSICBRAINZ_HXX
+
+extern const struct tag_table musicbrainz_txxx_tags[];
+
+#endif

src/tag/TagId3.cxx

@@ -20,6 +20,7 @@
 #include "config.h"
 #include "TagId3.hxx"
 #include "Id3Load.hxx"
+#include "Id3MusicBrainz.hxx"
 #include "TagHandler.hxx"
 #include "TagTable.hxx"
 #include "TagBuilder.hxx"
@@ -205,19 +206,8 @@ gcc_pure
 static TagType
 tag_id3_parse_txxx_name(const char *name) noexcept
 {
-	static constexpr struct tag_table txxx_tags[] = {
-		{ "ALBUMARTISTSORT", TAG_ALBUM_ARTIST_SORT },
-		{ "MusicBrainz Artist Id", TAG_MUSICBRAINZ_ARTISTID },
-		{ "MusicBrainz Album Id", TAG_MUSICBRAINZ_ALBUMID },
-		{ "MusicBrainz Album Artist Id",
-		  TAG_MUSICBRAINZ_ALBUMARTISTID },
-		{ "MusicBrainz Track Id", TAG_MUSICBRAINZ_TRACKID },
-		{ "MusicBrainz Release Track Id",
-		  TAG_MUSICBRAINZ_RELEASETRACKID },
-		{ nullptr, TAG_NUM_OF_ITEM_TYPES }
-	};
 
-	return tag_table_lookup(txxx_tags, name);
+	return tag_table_lookup(musicbrainz_txxx_tags, name);
 }
 
 /**