id3v2: Distinguish COMM frames by content descriptor

[uklotzde]

Plain comments are supposed to have an empty description aka content descriptor. Only those must appear as ItemKey::Comment, otherwise this would cause confusion and inconsistencies during the read/write round trip.

TODO

• How to filter the COMM frames such that only those with an empty content descriptor appear under ItemKey::Comment?
• Test that unsupported frames with a non-empty content descriptor are preserved during a read/write round trip. Out of scope, nothing to do here. Conversions from ID3v2Tag to Tag are expected to be lossy and applications are responsible for preserving unmapped MP3 metadata frames. All operations directly on ID3v2Tag are unaffected by the changes.

[Serial-ATA]

For the TODO, the ID3v2Tag -> Tag conversion could easily be changed to only accept empty descriptors here:

lofty-rs/src/id3/v2/tag.rs

Line 579 in 90a7683

FrameValue::Comment(LanguageFrame { content, .. })

The real issue is how should comments with a descriptor be stored? Currently, the descriptor just gets discarded, which definitely isn't ideal. Might have to take a similar route to the popularimeter, adding a new ItemKey and converting the LanguageFrame to binary.