* more widget const constructors

* prefer const constructors

* address review comments

Turns out we have many immutable classes.

Fixes #6892

Rename State.config to State.widget
Rename State.didUpdateConfig to State.didUpdateWidget
Renamed all State subclasses' local variables named config to something else

This fixes minor issues I ran into while using Flutter.

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.

I did a pass through some of the code cleaning minor things up.

Fixed a bug where the size of the AnimatedCrossFade would always
start from the size of the first child, irrespective of the
initial crossFadeState argument.

Added a widget that cross fades two children while animating the
size of the parent based on the children's interpolated sizes.