decoder/mp4ff: support more variations of "album artist"

According to the mantis bug report 2847, there are several possible
variations of the "album artist" tag:

- "album artist"
- "album_artist"
- "albumartist"

This patch adds support for the latter two.

src/decoder/mp4ff_decoder_plugin.c

@@ -362,6 +362,10 @@ mp4ff_tag_name_parse(const char *name)
 	if (type == TAG_NUM_OF_ITEM_TYPES)
 		type = tag_name_parse_i(name);
 
+	if (g_ascii_strcasecmp(name, "albumartist") == 0 ||
+	    g_ascii_strcasecmp(name, "album_artist") == 0)
+		return TAG_ALBUM_ARTIST;
+
 	return type;
 }