Add mappings for more MusicBrainz tags

[phw]

@LyricsLover

What's the difference between this MusicBrainz tags and TXXX tags in general?

I think this is similar to what was already changed for other MusicBrainz defined tags like MusicBrainz Artist Id. These are specifically defined tags with a specific meaning, and their casing should be preserved. MP3Tag already contains a fixed mapping of most of those tags, but not for the ones @culinko mentioned above. See also this older discussion:

Problem when entering MusicBrainz IDs (e.g., MusicBrainz Artist ID)

This is due to a change in v2.89 for the internal mapping of MusicBrainz IDs [2018-06-04] CHG: added mappings for special fields with MusicBrainz IDs for ID3v2 and MP4. (#18405) With this change, I've added an mapping from internal (written to the tag) to external (displayed in Mp3tag) representation of the MusicBrainz ID fields. This was necessary, because other applications (e.g., MusicBrainz Picard) or libraries (e.g., TagLib) are using different names for those identifiers depending on t…