MediaFile: More general comments field in ID3

[sampsyo]

An outgrowth of #1043, courtesy of @kiefermat.

Apparently, some software uses COMM frames with funky desc fields to store comments. (Most software uses an empty desc field.) We should probably read these various frames and interpret them as comments.

The question gets a little trickier when writing. I lean toward automatically "standardizing" these fields when MediaFile needs to update the tag—i.e., we should drop any frames with custom desc fields and just preserve the descless frame. That would require a bit more machinery than we currently have in MP3DescStorageStyle.

[Moonbase59]

That is true. For instance, using MusicBrainz Picard together with the last.fm.ng plugin gives all kinds of nice "comment" fields, like:

Songs-DB_Custom1 (decade, contains things like "2000s")
Songs-DB_Custom2 (contains things like "Classic; Male Vocalists")
Songs-DB_Custom3 (city the band comes from)
Songs-DB_Custom4 (country the band comes from)
Songs-DB_Occasion (contains things like "Party", "Christmas" or "Love; Romantic")

As far as I’ve seen, these are usually saved using the "eng" language setting for the frame. Multivalue properties shown here are dependent on the "ID3v2 separator" setting, I used "; ".

For other file types (think Vorbis Comments) these are stored somewhat differently), like:

2010s (Songs-DB_Custom1)
Female Vocalist (Songs-DB_Custom2)
Singer-Songwriter (Songs-DB_Custom2)
London (Songs-DB_Custom3)
British (Songs-DB_Custom4)
Love (Songs-DB_Occasion)
Chillout (Songs-DB_Occasion)

(taken from the Adele album "25").