tag/Type: add TAG_GROUPING

Map ID3 "TIT1" to this new core tag type.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/563

src/tag/Id3Scan.cxx

@@ -1,5 +1,5 @@
 /*
- * Copyright 2003-2018 The Music Player Daemon Project
+ * Copyright 2003-2019 The Music Player Daemon Project
  * http://www.musicpd.org
  *
  * This program is free software; you can redistribute it and/or modify
@@ -317,6 +317,7 @@ scan_id3_tag(const struct id3_tag *tag, TagHandler &handler) noexcept
 	tag_id3_import_text(tag, "TPE3", TAG_PERFORMER,
 			    handler);
 	tag_id3_import_text(tag, "TPE4", TAG_PERFORMER, handler);
+	tag_id3_import_text(tag, "TIT1", TAG_GROUPING, handler);
 	tag_id3_import_comment(tag, ID3_FRAME_COMMENT, TAG_COMMENT,
 			       handler);
 	tag_id3_import_text(tag, ID3_FRAME_DISC, TAG_DISC,

src/tag/Id3Scan.hxx

@@ -1,5 +1,5 @@
 /*
- * Copyright 2003-2018 The Music Player Daemon Project
+ * Copyright 2003-2019 The Music Player Daemon Project
  * http://www.musicpd.org
  *
  * This program is free software; you can redistribute it and/or modify

src/tag/Names.c

@@ -1,5 +1,5 @@
 /*
- * Copyright 2003-2018 The Music Player Daemon Project
+ * Copyright 2003-2019 The Music Player Daemon Project
  * http://www.musicpd.org
  *
  * This program is free software; you can redistribute it and/or modify
@@ -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_GROUPING] = "Grouping",
 	[TAG_COMMENT] = "Comment",
 	[TAG_DISC] = "Disc",
 	[TAG_LABEL] = "Label",

src/tag/Type.h

@@ -1,5 +1,5 @@
 /*
- * Copyright 2003-2018 The Music Player Daemon Project
+ * Copyright 2003-2019 The Music Player Daemon Project
  * http://www.musicpd.org
  *
  * This program is free software; you can redistribute it and/or modify
@@ -49,6 +49,7 @@ enum TagType
 	TAG_ORIGINAL_DATE,
 	TAG_COMPOSER,
 	TAG_PERFORMER,
+	TAG_GROUPING,
 	TAG_COMMENT,
 	TAG_DISC,
 	TAG_LABEL,