Replacing RefPtr with shared_ptr<>.

[Murray Cumming <murrayc murrayc com>]

For gtkmm 4 (well, in the corresponding glibmm), we are thinking of
replacing Glib::RefPtr with std::shared_ptr:
https://bugzilla.gnome.org/show_bug.cgi?id=755037

This is a large change so I'm mentioning it here too. It shouldn't
affect application code much if it works.

-- 
Murray Cumming
murrayc murrayc com
www.murrayc.com