cssnode: Handle invalidate queueing differently

Instead of directly requesting the layout phase, register a tick
callback. This is what the docs suggest for animations and it's what we
need for the next commit.

gtk/gtkcsswidgetnodeprivate.h

@@ -38,6 +38,7 @@ struct _GtkCssWidgetNode
   GtkCssNode node;
 
   GtkWidget *widget;
+  guint validate_cb_id;
 };
 
 struct _GtkCssWidgetNodeClass