Add a lock button widget

This is a special-purpose button that can be used together with
GPermission objects to control the sensitivity of system settings.

Suitable permission objects can e.g. be obtained from PolicyKit.

http://bugzilla.gnome.org/show_bug.cgi?id=626457

po/POTFILES.in

@@ -118,6 +118,7 @@ gtk/gtklabel.c
 gtk/gtklayout.c
 gtk/gtklinkbutton.c
 gtk/gtkliststore.c
+gtk/gtklockbutton.c
 gtk/gtkmain.c
 gtk/gtkmenubar.c
 gtk/gtkmenu.c