Extending TagLib::Tag's interface.

[Peter van Hardenberg]

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.

The Creative Commons guys are also interested in having easier ways to store and retrieve license information from files.

Patches will follow as I produce them.

-pvh