Adds itemsWrap:bool (default false) to ScrollableList and PageableList. If itemsWrap is true then scrolling past the last item wraps around to the first. Similarly, scrolling before the first item wraps around to the last.

Added abstract ExtentScrollBehavior of ScrollBehavior. Renamed fields called contentsExtents to contentExtent, containerExtents to containerExtent, contentSize to contentExtent, etc.

BoundedBehavior is now a subclass of ExtentScrollBehavior.

Added UnboundedBehavior subclass of ExtentScrollBehvaior; contentExtent and maxScrollOffset are double.INFINITY, minScrollExtent is double.NEGATIVE_INFINITY.

 - Introduce _setScrollOffset as a backend for the animations so that scrollTo
   can stop animations.

 - Create a single function that stops both kinds of scroll animations.

 - Refactor how we update the bounds for bounded scroll behaviors so that we
   update the bounds and compute the new scroll offset at the same time.

Used for AnimationPerformance with a single AnimatedValue<T> variable.

1. We would assert if you tried to start an animation from within an
animation status callback. This is a common pattern, so I fixed this
assert (in Ticker._tick).
2. We would assert for animations with duration under a millisecond.
Fixed.

Also removed the workarounds assocated with #1.

Just infer it from the sign of the velocity. That's what odeon does.

They settle much faster after you fling them now.

Also add intervals and curves to AnimationPerformance, which affect all
variables used by the performance.

This ensures we don't run into discontinuities when reversing an
animation halfway through. I refactored AnimationValue to have knowledge
of the reverse curves and intervals.

Simplifies the code a bit.

When a ScrollableBlock can scroll, we want to be able to drag into the
overscroll region. Previously we could fling into the overscroll region, but we
couldn't actually drag there.

Use those in SnackBar, Drawer, navigator instead of AnimatedContainer's
intentions.

This required some changes to AnimationPerformance to better understand
animating with forces.

SnackBar, and PopupMenu instead of custom statuses.

It turns out that we aren't really using these. The identity logic is sufficient.

Also, add some asserts for a crash I had once but couldn't reproduce, in case that helps catch it next time.