avformat/id3v2: Add Grouping frame to id3v2.4 metadata table

The ID3v2.4.0 standard defines TIT1 as the "Content group description"
tag [1]. This frame is usually referred to as the "Grouping" tag and in
de-facto use under that name by Vorbis and APEv2 [2].

This commit introduces a mapping from "TIT1" to "grouping" in the
id3v2.4 metadata conversion table. This will enable software to access
it using that name. In particular, MPD will now read this tag correctly
when using the ffmpeg decoder plugin.

[1] https://id3.org/id3v2.4.0-frames (4.2.1)
[2] https://picard-docs.musicbrainz.org/en/appendices/tag_mapping.html#grouping-3

Signed-off-by: Wolfgang Müller <wolf@oriole.systems>
Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavformat/id3v2.c

@@ -69,6 +69,7 @@ const AVMetadataConv ff_id3v2_4_metadata_conv[] = {
     { "TSOA", "album-sort"    },
     { "TSOP", "artist-sort"   },
     { "TSOT", "title-sort"    },
+    { "TIT1", "grouping"      },
     { 0 }
 };