appDisplay: Don't show page indicators if there's only one page

It doesn't make sense to show the indicators in that case, so
don't show them. This has been the design in the first place,
but the code that did that was lost at some point during review ...

https://bugzilla.gnome.org/show_bug.cgi?id=707363

js/ui/appDisplay.js

@@ -224,6 +224,7 @@ const PageIndicators = new Lang.Class({
                 children[i].destroy();
         }
         this._nPages = nPages;
+        this.actor.visible = (this._nPages > 1);
     },
 
     setCurrentPage: function(currentPage) {