widget: Add a hook for resizes

It's private, no APIs, we don't talk about it. But we will start using
it very soon, so we can do size request caching in columns and avoid
sizegroups...

gtk/gtkwidget.c

@@ -3555,6 +3555,9 @@ gtk_widget_queue_resize_internal (GtkWidget *widget)
   priv->resize_needed = TRUE;
   gtk_widget_set_alloc_needed (widget);
 
+  if (priv->resize_func)
+    priv->resize_func (widget);
+
   groups = _gtk_widget_get_sizegroups (widget);
 
   for (l = groups; l; l = l->next)

gtk/gtkwidgetprivate.h

@@ -122,6 +122,7 @@ struct _GtkWidgetPrivate
 
   int width_request;
   int height_request;
+  void (* resize_func) (GtkWidget *);
   GtkBorder margin;
 
   /* Animations and other things to update on clock ticks */