Improve metadata fields exposed

[ohrenkino]

@MotleyG

view or handle these PRIV fields

In the meantime, a further property can be displayed - the presence of unknown frames.
Please see this thread:

%_id3v2_unknown_frames% - which are they?

I made a list of frames which are mentioned in the ID3V2 standard but which are not supported by MP3tag: ID3-Id Description AENC Audio encryption COMR Commercial frame CTOC Table of contents The purpose of "CTOC" frames is to allow a table of contents to be defined. ENCR Encryption method registration EQUA Equalization ETCO Event timing codes GEOB General encapsulated object GRID Group identification registration LINK Linked information MCDI Music CD identifier MLLT M…

And not all frames should be deleted or are "unwanted", e.g. some podcasts come with CHAP frames which are interpreted by dedicated podcast players.