Expose isPlaying

[siwel]

It can be useful to know if the tween is started, while the user could keep track of this themselves we already track this so I have added a getter to expose this.

[mikebolt]

I think there's an issue with _isPlaying. It will be true after a tween finishes. Once that issue is fixed I can merge this.

[mikebolt]

I made a small change to fix that issue, but there is still another issue. isPlaying() will return true after .start() is called, but before the tween actually starts, if a later start time is specified. It is not straightforward to fix this because the stop() method relies on _isPlaying and the stop() method is supposed to work even before the tween actually starts.