PopupSwitchMenuItem: allow toggling without closing the menu

Similar to what Gtk does, now toggling with Space does not close
the menu.

https://bugzilla.gnome.org/show_bug.cgi?id=664416

js/ui/popupMenu.js

@@ -774,6 +774,12 @@ const PopupSwitchMenuItem = new Lang.Class({
             this.toggle();
         }
 
+        // we allow pressing space to toggle the switch
+        // without closing the menu
+        if (event.type() == Clutter.EventType.KEY_PRESS &&
+            event.get_key_symbol() == Clutter.KEY_space)
+            return;
+
         this.parent(event);
     },