Allow `cancelTweensOf` to cancel tweens on child properties

[Geokureli]

FlxTween allows tweens on child properties of the supplied object, like the following:

FlxTween.tween(camera, {'scroll.y': 500}, 1.0);

To cancel the tween above, one needs to do the following:

hxFlxTween.cancelTweensOf(camera.scroll);

I suggest that because the tween was added to the camera, we should allow devs to cancel it via:

hxFlxTween.cancelTweensOf(camera);

The tween was added to the camera, specifically, so it's natural to think that it can be cancelled the same way. The tween has a reference to both camera.scroll and camera, so it's not difficult, and I don't think this will introduce any new confusions with cancelling tweens.

I'll have a PR soon (been busy/lazy, lately) but I plan to just check if (_object == object) in this function