dash: Do not shadow ClutterActor's destroy()

Since commit ef1e27966d turned DashItemContainer into an StWidget,
the destroy() method overrides the ClutterActor method, which is at
the very least bad style. Instead, follow the usual pattern of using
a ::destroy handler.

https://bugzilla.gnome.org/show_bug.cgi?id=791233

js/ui/dash.js

@@ -50,6 +50,10 @@ var DashItemContainer = new Lang.Class({
         this._childScale = 0;
         this._childOpacity = 0;
         this.animatingOut = false;
+
+        this.connect('destroy', () => {
+            this.label.destroy();
+        });
     },
 
     vfunc_allocate: function(box, flags) {
@@ -177,11 +181,6 @@ var DashItemContainer = new Lang.Class({
                          });
     },
 
-    destroy: function() {
-        this.label.destroy();
-        this.parent();
-    },
-
     animateOutAndDestroy: function() {
         this.label.hide();