St: don't attempt to give focus to non reactive actors

Non reactive actors don't expect to be interacted with, and thus
should not get keyboard focus.

https://bugzilla.gnome.org/show_bug.cgi?id=619955

src/st/st-widget.c

@@ -1880,7 +1880,8 @@ st_widget_real_navigate_focus (StWidget         *widget,
   while (focus_child && clutter_actor_get_parent (focus_child) != widget_actor)
     focus_child = clutter_actor_get_parent (focus_child);
 
-  if (widget->priv->can_focus)
+  if (clutter_actor_get_reactive (widget_actor) &&
+      widget->priv->can_focus)
     {
       if (!focus_child)
         {