Mostly related to semantics and scrolling.

This patch adds docs for many of the sliver widgets.

The new name follows the pattern of the name suggesting which layout
protocol the parent expects the child to speak.

Fixes #8664

Also make locals final where possible.

Followup to 52715467.

* Improved defaults for scroll view primary-ness

* Vertical scroll views default to primary:true.
* Horizontal scroll views default to primary:false.
* If a scroll view is primary and it got a non-null inherited primary
  scroll controller, it introduces a primary scroll controller inherited
  with a value of null for its descendants.

ScrollController now multiplexes writes to all registered positions;
reads of position continue to assert that only one position is
registered. Reads still require a single position.

Also a small formatting fix.

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.

Move the back button and drawer opening logic into the app bar.

Move the tap-status-bar-to-scroll-to-top logic to using
ScrollControllers. Provide a PrimaryScrollController and a `primary`
flag on scroll views.

Make it possible to track when a route becomes or stops being poppable.

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.

If you pass a ScrollController to a Scrollable2, you can use the controller to
read and write the scroll offset without having to find the Scrollable2State
object.

This patch uses sliver-based two more gallery demos, the stocks example,
in the date picker, and in markdown.

Also, use CustomScrollView in Shrine and fix a bug with one-line grids
not painting properly due to their reporiting zero paintExtent.

This prepares us for a CustomScrollView that takes slivers.

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 property is wired up to anything and it isn't used. We'll need to find a
better way for clients to control the scroll offset.

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.

The only remaining client of ScrollableGrid (the old version) is Pesto,
which needs AppBar integration.

This patch adds grid supports to slivers and introduces a ScrollGrid
convenience class for making the common types of scrollable grids.

This patch also deploys ScrollGrid in an example in the Flutter Gallery.

Add SliverList

A SliverList is a linear layout of box children in a viewport that all
have a common, fixed extent along the scroll axis. The layout is similar
to a SliverBlock but more efficient.