Adds a widget to the list of mnemonic labels for
this widget. (See gtk_widget_list_mnemonic_labels()). Note the
list of mnemonic labels for the widget is cleared when the
widget is destroyed, so the caller must make sure to update
its internal state at this point as well, by using a connection
to the #GtkWidget::destroy signal or a weak notifier.
Adds a widget to the list of mnemonic labels for this widget. (See gtk_widget_list_mnemonic_labels()). Note the list of mnemonic labels for the widget is cleared when the widget is destroyed, so the caller must make sure to update its internal state at this point as well, by using a connection to the #GtkWidget::destroy signal or a weak notifier.