network: Fix all Wi-Fi networks having gear buttons

Only the networks with existing connection should have gear buttons.
This is a regression from 9ffaff7472.

If the widget we add as a stack page isn't visible, the page will not be
shown. As we later add another page with that button, we'd end up
showing that by default.

https://bugzilla.gnome.org/show_bug.cgi?id=750841

panels/network/net-device-wifi.c

@@ -1555,6 +1555,7 @@ make_row (GtkSizeGroup   *rows,
         gtk_widget_show (button_stack);
 
         widget = gtk_label_new ("");
+        gtk_widget_show (widget);
         gtk_container_add (GTK_CONTAINER (button_stack), widget);
 
         widget = NULL;