Don't mix GJS and GObject signal systems

GJS implements a basic signal system that allows monkey-patching
JS objects with signal methods resembling the GObject ones. However
it's clearly not a good idea to replace the actual GObject methods,
so use the proper GObject facilities when inheriting from GObject.

https://bugzilla.gnome.org/show_bug.cgi?id=778660

js/ui/edgeDragAction.js

@@ -14,6 +14,7 @@ const DRAG_DISTANCE = 80;
 const EdgeDragAction = new Lang.Class({
     Name: 'EdgeDragAction',
     Extends: Clutter.GestureAction,
+    Signals: { 'activated': {} },
 
     _init : function(side, allowedModes) {
         this.parent();
@@ -81,4 +82,3 @@ const EdgeDragAction = new Lang.Class({
             this.emit('activated');
     }
 });
-Signals.addSignalMethods(EdgeDragAction.prototype);