An object containing the different Tween callback functions.
You can either set these in the Tween config, or by calling the
onComplete When the Timeline finishes playback fully or
Timeline.stop is called. Never invoked if timeline is set to repeat infinitely.
onLoop When a Timeline loops.
onStart When the Timeline starts playing.
onUpdate When a Timeline updates a child Tween.
onYoyo When a Timeline starts a yoyo.