MediaFile: More general comments field in ID3

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 `desc`less frame. That would require a bit more machinery than we currently have in `MP3DescStorageStyle`.

Start by reading the existing ID3 comment handling and MP3DescStorageStyle, then review the context in issue #1043. Determine how COMM frames with custom desc fields should be read and how updates should standardize them while preserving the desc-less frame; done means both behaviors are covered consistently.

Written by the indexing model from the issue text.