We need a way to indicate to gnome-control-center that we want the
keybindings capplet to show the Window Manager keybindings for Metacity;
do this through a _GNOME_WM_KEYBINDING property we put on the
_NET_SUPPORTING_WM_CHECK window and set to Mutter,Metacity.
See http://bugzilla.gnome.org/show_bug.cgi?id=594066 for the
gnome-control-center part of this.
http://bugzilla.gnome.org/show_bug.cgi?id=594067
0b86343dd0
