Rename all the "2" classes related to scrolling to remove the "2". Now that the
old scrolling code is gone, we don't need to use the suffix.

Also, rename ScrollableMetrics to ScrollMetrics, which follows the
naming convention for most of the other classes (e.g., ScrollPosition,
ScrollPhysics).

Finally, fix a bug whereby SingleChildScrollView could not have a
GlobalKey, because, write test, find bug.

This patch improves PageView to the point where we can use it in the date
picker. Specifically, you now get onPageChanged notifications and you can
control which page is visible using a PageController.

Also, move the creation of the ScrollPosition to ScrollController.

Finally, remove TestScrollable in favor of CustomScrollView, which is the
production version of this widget.

This patch makes Scrollable2.ensureVisible with SingleChildScrollView. A future
patch will extend the implementation to work with slivers. (Although the patch
does include some of the infrastructure for that part of the implementation as
well.)

This patch makes a number of changes to how you can configure a
Scrollable2:

 - The ScrollPhysics is now responsible for creating the ScrollPosition.
   You can override the ScrollPhysics by supplying a `physics` argument
   to `Scrollable`, and the new physics you supply will be applied to
   the default physics inherited from the ScrollBehavior.

 - This patch removes the ScrollPosition/AbsoluteScrollPosition split as
   all clients were operating in pixels anyway and the split made the
   code very difficult to follow.

 - ScrollPosition no longer depends directly on Scrollable2State.
   Instead, it depends on an abstract interface that Scrollable2State
   implements. This change has two benefits:

    a) It removes the circular dependency between ScrollPosition and
       Scrollable2State, which lets us split the code for these classes
       (and several other classes that got wrapped up in that cycle) into
       separate libraries for easier maintenance.

    b) ScrollPosition is no longer bound to Scrollable2, which means you
       could use the behavior machinery to drive other sorts of widgets.
       For example, we could use it to drive Scrollabe1 if we wanted.

This widget is a start towards replacing PageableList. There are still a number
of features that we'll need to add before this widget can replace PageableList.

This structure makes it easier to customize the viewport used in a Scrollable.
Now Scrollable2 doesn't commit to using slivers.

This widget is a replacement for ScrollableViewport that uses the new
Scrollable2 machinery. The widget is not based on Slivers but does use the new
scroll behavior classes.