Fixes #6591

Mostly around pointing people towards Block more aggressively.

* ClampOverscrolls Inherited Widget

* Added OverscrollIndicator, removed OverscrollIndicatorPainter

The dartdoc will continue until morale improves.

* Added Scrollbar, removed ScrollbarPainter

* removed a dead import

* updated per review feedback

* Only call dispatch if the widgets State is still mounted

Overscroll indicator for MaterialList

This patch renames StatelessComponent to StatelessWidget and
StatefulComponent to StatefulWidget.

Fixes #2308

Fixes #1382

These files are already in the "material" folder. They don't need to be
prefixed with material again.

Fixes #702

Add support for the appbar behavior described in the "Flexible space with image" section of https://www.google.com/design/spec/patterns/scrolling-techniques.html#scrolling-techniques-scrolling.

We use the ScrollDirection for more than just scrolling.

Fixes #151

ScrollableList2 is complete and subsumes all the use cases for the original
ScrollableList.

This patch also changed ScrollableList2 to use an Iterable instead of an
List for its children. This change lets clients map their underlying
data lazily. If the clients actually have a concrete list, we skip the
extra copy and grab the child list directly.

A MaterialList understands the sizing, padding, and scrollbar features of
Material Design lists.

Also, add CircleAvatar for showing the circular avatars that are commonly used
in material lists.