Add TT methods to StateModififer

The current implementation of the StateModifier uses a TransitionableTransform, but exposes only its set method.

I often experienced the situation in which I wanted to animate multiple transforms at the same time, which is pretty annoying in the current implementation, since the StateModifier only exposes a setTransform() method.

## As a concrete example: Rotating and translating at the same time with different easing curves
```javascript
stateModifier.setRotate([Math.PI*.5, Math.PI*.5, Math.PI*.5], {duration: 5000, Easing.inBounce});
stateModifier.setTranslate([100, 200, 300], {duration: 5000, Easing.inBounce});
```

I think this is much more user-friendly than the current approach of setting the transform e.g. via Transform.translate().

Things that would need to be discussed: Now that there are setters, should there also be getters. If so, should the getters return the final or current state of the transition? Or both?

Thoughts on that?