GtkToolButton::set_label_widget
void set_label_widget(GtkWidget label_widget);
Sets label_widget as the widget that will be used as the label for button. If label_widget is NULL the "label" property is used as label. If "label" is also NULL, the label in the stock item determined by the "stock_id" property is used as label. If "stock_id" is also NULL, button does not have a label.
See also: get_label_widget()