overview: Fix stuck grabs when mashing the overlay-key

When pressing the overlay key three times, things went like this:

  * show(), push a modal
  * hide(), will pop a modal after hiding is done
  * show(), push a modal

Thus, when the showing is done, and then it activated the hiding,
it popped one modal, but not the other. This patch changes things
to be:

  * show(), push a modal
  * hide(), will pop a modal after hiding is done
  * hide(), no-op

That is, mashing the overlay-key when it's showing will always make
it hide, not mashing an odd number of times.

https://bugzilla.gnome.org/show_bug.cgi?id=688589

js/ui/overview.js

@@ -562,7 +562,7 @@ const Overview = new Lang.Class({
         if (this.isDummy)
             return;
 
-        if (this._shown)
+        if (this.visible)
             this.hide();
         else
             this.show();