js: Use EVENT_PROPAGATE/EVENT_STOP constants in event handlers

Just as SOURCE_CONTINUE/SOURCE_REMOVE in source functions, these
constants increase code clarity over plain true/false.

https://bugzilla.gnome.org/show_bug.cgi?id=719567

js/ui/boxpointer.js

@@ -69,7 +69,7 @@ const BoxPointer = new Lang.Class({
     _muteInput: function() {
         if (this._capturedEventId == 0)
             this._capturedEventId = this.actor.connect('captured-event',
-                                                       function() { return true; });
+                                                       function() { return Clutter.EVENT_STOP; });
     },
 
     _unmuteInput: function() {