Sets label_widget as the widget that will be used as the label
for button. If label_widget is null the #GtkToolButton:label property is used
as label. If #GtkToolButton:label is also null, the label in the stock item
determined by the #GtkToolButton:stock-id property is used as label. If
#GtkToolButton:stock-id is also null, button does not have a label.
Sets label_widget as the widget that will be used as the label for button. If label_widget is null the #GtkToolButton:label property is used as label. If #GtkToolButton:label is also null, the label in the stock item determined by the #GtkToolButton:stock-id property is used as label. If #GtkToolButton:stock-id is also null, button does not have a label.