Add showModalStopper to exploit the provided showModalBottomSheet function

[savy-91]

These commits add a showModalStopper function that uses the showMobalBottomSheet flutter function to show a modal bottom sheet, possibly with rounded corners and without the possibility to interact with the background. It should also close https://github.com/aryzhov/flutter-stopper/issues/1

[Praveen-Pandiyan]

Exactly what i want 😃