Add support for new iTunes Grouping Tag (ID3v2)

[mixxxbot]

Reported by: uklotzde Date: 2018-05-27T17:45:50Z Status: New Importance: Undecided Launchpad Issue: lp1773684 Tags: metadata

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Apple has (silently?) changed the mapping for the Grouping field to ID3v2 tags in iTunes 12.5.4.42:

https://community.mp3tag.de/t/work-name-name-and-movement/18175 https://community.mp3tag.de/t/work-grouping-fields-itunes-12-5/18659 https://community.mp3tag.de/t/support-for-new-itunes-grouping-id3-frame-grp1/18523

http://mutagen.readthedocs.io/en/latest/api/id3_frames.html#mutagen.id3.GRP1 https://github.com/quodlibet/mutagen/issues/304 https://github.com/quodlibet/mutagen/commit/d7f7ef008eed860c0e4909e32cc6a353cb9a0669

This conflicts with the mapping to "Content group description" as proposed by MusicBrainz Picard: https://picard.musicbrainz.org/docs/mappings/

We need a concept how to deal with those differences.

Import ID3v2 tags: Prioritize MusicBrainz over Apple

• Read Grouping from TIT1 frame first.
• Read Grouping from GRP1/GP1 frame if TIT1 frame does not exists or content is empty.

Export ID3v2 tags: Try to avoid redundant information in multiple frames

• Write Grouping into GRP1 if the frame already exists, otherwise skip it.
• Write Grouping into TIT1 frame if the frame already exists. Add it only if no GRP1 frame has been written (see above).

[mixxxbot]

Duplicate of mixxxdj/mixxx#9315