Better overscroll / overpinch - replace thresholds with friction

[natario1]

Currently the engine lets you scroll / drag / pinch outside the safe bounds, until a fixed threshold is reached.

This does not feel very natural. The best thing would be to have no threshold, but rather apply a friction to the movement (the more you overdrag, the less movement you cause).

These frictions should be public and controllable.

[natario1]

This was done in #59 for overscroll, but it's not so elegant and does not expose parameters to public. Still missing for overflings