AppMenuButton: Don't set the actor reactive if it's not really visible

Reactive means that the actor is reachable from keyboard
navigation. If the target isn't current that means we are not tweening
the actor to be visible so we shouldn't set it reactive either.

https://bugzilla.gnome.org/show_bug.cgi?id=671998

js/ui/panel.js

@@ -317,11 +317,12 @@ const AppMenuButton = new Lang.Class({
 
         this._visible = true;
         this.actor.show();
-        this.actor.reactive = true;
 
         if (!this._targetIsCurrent)
             return;
 
+        this.actor.reactive = true;
+
         Tweener.removeTweens(this.actor);
         Tweener.addTween(this.actor,
                          { opacity: 255,