core/objects/icon: switch to Gtk+ 3.0.

This resolves the gobject-related conflicts in the logout_vera module.
1.23.1
Merge branch 'devel' into debian-devel
debian/rules: do not guess python dependencies.
debian/control: depend on python-gi and gir1.2-gtk-3.0; removed python-t9n and python-gtk2.