cancel_button
Description:
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 [property@Gtk.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.