experimental 2.3.x support in CVS

[muppet <scott asofyet org>]

Now that the necessary versioning infrastracture is in place in HEAD,
i've added experimental support for gtk+-2.3.x under the branch tag
"unstable-2-3-x-branch". Play with it if you dare!  (and if you do,
please submit bug reports. :)

This is more than the few files included in the Gtk2::3 module i posted
a week or two ago --- this branch has 20 or so new XS files and adds a
slew of new methods to many existing objects.  In addition to the new
file chooser and combo box, there's the new action-oriented toolbar API,
the UIManager (xml definitions for menu items), and a nifty expander
widget.

"Why is this code not in HEAD?" you ask...  because 2.3.x is not
guaranteed to be api stable.  However, having all this in CVS means
we'll be able to get out a release for 2.4.0 in no time flat.