Flickable: add a property to measure amount of overshoot even when overshoot is disabled

See https://material.google.com/patterns/swipe-to-refresh.html#swipe-to-refresh-usage for a possible use case. To implement that, you'd need to have boundsBehavior: Flickable.StopAtBounds but also somehow detect how far the user has tried to go past the bounds. Overshooting at one end could result in negative values, and positive values at the other end, so you can tell which way it's being dragged.