Support "album artist" and "artist sort" in TagLib::Tag interface?

[Scott Wheeler]

Ralf Engels wrote:
> My proposal:
> for now let's stay with the methods but at least implement some dummy
> functions in TagLib::Tag.
>   

The problem is that adding virtual functions to an interface isn't 
binary compatible, so it can't be done that way within the TagLib 1.5 
series.  Peter's done a lot of work on a more extensible API that we'll 
probably use some version of for 2.0.

-Scott