tag/Type: add tag "Work"

Closes https://github.com/MusicPlayerDaemon/MPD/issues/577

NEWS

@@ -3,7 +3,7 @@ ver 0.22 (not yet released)
   - "findadd"/"searchadd"/"searchaddpl" support the "sort" and
     "window" parameters
 * tags
-  - new tag "Grouping" (for ID3 "TIT1")
+  - new tags "Grouping" (for ID3 "TIT1") and "Work"
 * input
   - ffmpeg: allow partial reads
 * archive

doc/protocol.rst

@@ -274,6 +274,8 @@ The following tags are supported by :program:`MPD`:
 * **date**: the song's release date. This is usually a 4-digit year.
 * **composer**: the artist who composed the song.
 * **performer**: the artist who performed the song.
+* **work**: `"a work is a distinct intellectual or artistic creation,
+  which can be expressed in the form of one or more audio recordings" <https://musicbrainz.org/doc/Work>`_
 * **grouping**: "used if the sound belongs to a larger category of
   sounds/music" (`from the IDv2.4.0 TIT1 description
   <http://id3.org/id3v2.4.0-frames>`_).

src/tag/Names.c

@@ -34,6 +34,7 @@ const char *const tag_item_names[TAG_NUM_OF_ITEM_TYPES] = {
 	[TAG_ORIGINAL_DATE] = "OriginalDate",
 	[TAG_COMPOSER] = "Composer",
 	[TAG_PERFORMER] = "Performer",
+	[TAG_WORK] = "Work",
 	[TAG_GROUPING] = "Grouping",
 	[TAG_COMMENT] = "Comment",
 	[TAG_DISC] = "Disc",

src/tag/Type.h

@@ -49,6 +49,7 @@ enum TagType
 	TAG_ORIGINAL_DATE,
 	TAG_COMPOSER,
 	TAG_PERFORMER,
+	TAG_WORK,
 	TAG_GROUPING,
 	TAG_COMMENT,
 	TAG_DISC,