Returns the default action for the current class.
- iOS 2.0+
- macOS 10.5+
- Mac Catalyst 13.0+
- tvOS 9.0+
- Core Animation
The identifier of the action.
Returns a suitable action object for the given key or
nil of no action object was associated with that key.
Classes that want to provide default actions can override this method and use it to return those actions.