Extending TagLib::Tag's interface.

[Jeff Mitchell]

Peter van Hardenberg wrote:
> We are using TagLib::Tag at Songbird for prototyping our metadata writing functionality and of course, it only supports a few of the most basic fields. I'd like to extend this list of fields to include more of the set of properties which are defined in ID3v2 and widely accepted by the Vorbis comment/APE community such as "composer". 
> 
> I would also like to implement an "extended(propName)" / "setExtended(propName, value)" function pair to provide a more general metadata storage for custom properties. I'm open to suggestions on how to implement this, but I think that for xiphcomment and ape, this would mostly be a matter of getting/setting the upcased value. For ID3v2, I was considering following iTunes' example and creating a comment with that fieldname as description.

Consider doing this properly and using e.g. User-Defined Text 
Information frames as in part 4.2.6 of the id3v2.4 standard.

Just because iTunes doesn't follow the standard doesn't mean you 
shouldn't either...   :-)

--Jeff