Please display RVA2 desc field

Thank you for kid3! I've been a happy user for years.

One thing I am currently missing: RVA2 identification field. If a file has multiple RVA2 tags, they all show up in the UI as generic 'Volume Adjustment', despite having a text descriptor field. For example, I have some tracks with three annotations: RVA2:track, RVA2:album (both from quodlibet replaygain plugin), RVA2:SeratoGain (from manual gain adjustment in Serato DJ). It would be nice to be able to visually differentiate between those in kid3 UI, as is done for APIC types. This could be done for UFID fields as well, I have several different types in my collection, but the content is strange for some of them (I think beatport ships invalid data here)

To create a reproducing file on a linux distro, it's fairly easy: install quodlibet, enable the 'replaygain' plugin, add a demo file to quodlibet UI, right click on the file, plugins->replaygain, process the song and save the content, it should now have RVA2:track and RVA2:album annotations.

Imported from SourceForge, author: aintdiscole