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

[Ralf Engels]

Hi Jason,
I thought about adding two other attributes.

RelativeGain and Popularimeter

Both of them are used or should be used in almost all Linux MP3 players.
e.g. gtkpod has it's own elaborate implementation to determine the audio
gain via the four different methods that are supported by the different
tags.

Adding them would be a no-brainer but as you already noticed one would
need to add an implementation to all tags.

I would propose that TagLib::Tag implements a dummy function for this
meta informations.
Then we would be free to add more without further problems.
And you could add your album artists.

BR,
Ralf