This property determines what happens when the Escape key is pressed while the alert is shown.
If this property holds the index of a button in gtk.alert_dialog.AlertDialog.buttons, then pressing Escape is treated as if that button was pressed. If it is -1 or not a valid index for the buttons array, then an error is returned.
If buttons is NULL, then the automatically created 'Close' button is treated as both cancel and default button, so 0 is returned.
Builder instance for fluent chaining
Set cancelButton property.