tag_ape: support album artist, MusicBrainz tags

I took these tag names from a MusePack sample file I got from a user.
These are not documented in the APE specification:

 http://wiki.hydrogenaudio.org/index.php?title=APE_key

People seem to be using undocumented extensions to the specification
anyway, and the best we can do is attempt to support them.

NEWS

@@ -20,6 +20,7 @@ ver 0.16 (20??/??/??)
 * tags:
   - added tags "ArtistSort", "AlbumArtistSort"
   - id3: revised "performer" tag support
+  - ape: support album artist, MusicBrainz tags
 * decoders:
   - don't try a plugin twice (MIME type & suffix)
   - don't fall back to "mad" unless no plugin matches

src/tag_ape.c

@@ -29,11 +29,18 @@
 static const char *const ape_tag_names[] = {
 	[TAG_TITLE] = "title",
 	[TAG_ARTIST] = "artist",
+	[TAG_ARTIST_SORT] = "artistsort",
 	[TAG_ALBUM] = "album",
+	[TAG_ALBUM_ARTIST] = "album artist",
+	[TAG_ALBUM_ARTIST_SORT] = "albumartistsort",
 	[TAG_COMMENT] = "comment",
 	[TAG_GENRE] = "genre",
 	[TAG_TRACK] = "track",
-	[TAG_DATE] = "year"
+	[TAG_DATE] = "year",
+	[TAG_MUSICBRAINZ_ARTISTID] = "musicbrainz_artistid",
+	[TAG_MUSICBRAINZ_ALBUMID] = "musicbrainz_albumid",
+	[TAG_MUSICBRAINZ_ALBUMARTISTID] = "musicbrainz_albumartistid",
+	[TAG_MUSICBRAINZ_TRACKID] = "musicbrainz_trackid",
 };
 
 static struct tag *