ItemKey: Add ItemKey::AlbumArtists #522
Description
Summary
As a analog to ItemKey::TrackArtists as was added in #454.
From what i can tell, it follows the same pattern as TXX:ARTISTS, but at least currently i dont see much support for this tag (for example in picard).
Kodi seems to support this as TXX:ALBUMARTISTS for Id3v2 (and others).
API design
A new key on ItemKey named AlbumArtists.