ScrollBehavior refactor

When using an OverscrollIndicator, have the corresponding ScrollBehavior actually clamp at min/max offset. Using an OverscrollIndicator doesn't let you reverse direction immediately #5226
When the ScrollBehavior has clamped, it should fire an "overscroll" notification saying by how much more it would have scrolled if it hadn't in fact clamped.
OverscrollIndicator should use that to grow itself / darken itself. OverscrollIndicator doesn't get darker as you get higher #5274 OverscrollIndicator doesn't flatten out when it goes away (it scrolls away instead) #5275
OverscrollIndicator should drive all its animations of its own animation controller, not the scroll offset. OverscrollIndicator grows suddenly #5279
OverscrollIndicator should paint based on the scroll offset, though. OverscrollIndicator doesn't paint at the bottom of the contents (it paints at the bottom of the container) #5276
ScrollBehavior doesn't need to be parameterised. It is always one dimensional and always uses doubles. ScrollBehavior doesn't need to be parameterised. It is always one dimensional and always uses doubles. #6092

This doesn't help #5277.

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