Starts a Tween playing.
You only need to call this method if you have configured the tween to be paused on creation.
If the Tween is already playing, calling this method again will have no effect. If you wish to
restart the Tween, use Tween.restart instead.
Calling this method after the Tween has completed will start the Tween playing again from the beginning.
This is the same as calling Tween.seek(0) and then Tween.play().
This Tween instance.