Flags the Tween as being complete, whatever stage of progress it is at.
If an onComplete callback has been defined it will automatically invoke it, unless a
argument is provided, in which case the Tween will delay for that period of time before calling the callback.
If you don't need a delay, or have an onComplete callback, then call
The time to wait before invoking the complete callback. If zero it will fire immediately.
This Tween instance.