Make listeners work

[joachimbulow]

Is your feature request related to a problem? Please describe.
It is hard to make custom functionality that depends on sheet state work without the listeners emitting values.

Describe the solution you'd like
When the controller animation value changes, please let it emit values so we can attach listeners.
E.g. i want to blur out the lowerLayer, which i cannot really do while the listeners do not work, without manual work.

Really makes the library almost useless

E.g.

  @override
  void initState() {
    super.initState();
    controller = RubberAnimationController(
      vsync: this,
      lowerBoundValue: AnimationControllerValue(pixel: 0),
      upperBoundValue: AnimationControllerValue(percentage: 0.9),
      duration: Duration(milliseconds: 200),
    );

    controller.addListener(() {
      // Do my custom stuff
    });

[joachimbulow]

Nvm