This requires all AnimationController objects to be given a
TickerProvider, a class that can create the Ticker.

It also provides some nice mixins for people who want to have their
State provide a TickerProvider. And a schedulerTickerProvider for those
cases where you just want to see your battery burn.

Also, we now enforce destruction order for elements.

Fixes #5393.

And make Scrollbar work with LazyBlock.

And an about box to the Stocks sample app.

This patch completes the circuit on this library. The next step is to go
through and add docs to anything we missed.

* Added Scrollbar, removed ScrollbarPainter

* removed a dead import

* updated per review feedback

* Only call dispatch if the widgets State is still mounted