[gtkmm] gtkmm has branched for gtkmm 2.5/2.6

["Murray Cumming" <murrayc murrayc com>]

I have branched the gtkmm module, so that we can start adding API in the
HEAD branch, such as the API in GTK+ 2.5/2.6. I haven't actually started
to wrap anything new yet though.

As always, full branch details are here:
http://www.gtkmm.org/cvs.shtml

Murray Cumming
murrayc murrayc com
www.murrayc.com
www.openismus.com