MoreLink: Ignore calls to activate() if the actor isn't visible

This fixes (another time) the bug about the "Recent Items" label
being clickable when there are no items.

https://bugzilla.gnome.org/show_bug.cgi?id=582037

js/ui/dash.js

@@ -583,6 +583,9 @@ MoreLink.prototype = {
     },
 
     activate: function() {
+        if (!this.actor.visible)
+            return true; // If the link isn't visible we don't want the header to react
+                         // to clicks
         if (this.pane == null) {
             // Ensure the pane is created; the activated handler will call setPane
             this.emit('activated');