Out of curiosity I noticed that some widgets that implement the GObjectClass::notify vfunc are not hooking up to the parent class implementation, shall it exist. This is unlikely to cause trouble if the ancestor classes have no properties or don't implement this vfunc, but I'd say they should be there in any case. Check GtkToolItem for instance:

static void
gtk_tool_item_property_notify (GObject    *object,
			       GParamSpec *pspec)
{
  GtkToolItem *tool_item = GTK_TOOL_ITEM (object);

  if (tool_item->priv->menu_item && strcmp (pspec->name, "sensitive") == 0)
    gtk_widget_set_sensitive (tool_item->priv->menu_item,
			      gtk_widget_get_sensitive (GTK_WIDGET (tool_item)));
}