There should be no more uses of Performance or AnimatedValue in the framework
or the examples.

Providing a pre-built child is more efficient because we don't need to rebuild
the child every tick of the animation.

Also, clean up the class hierarchy for AnimationController now that
we've renamed progress to value. That means everything in the hierarchy
now has a value, include Watchable. This patch renames Watchable to
Animated<T>, which lets us use that type almost everywhere.

I've added some ducktape to modal bottom sheets to avoid having to
refactor all of Navigator to use AnimationController. I'll remove the
ducktape in the next patch.

These now use Animation and AnimationController instead of PerformanceView and
Performance.

Introduces a new Tooltip class.
Adds support for tooltips to IconButton and Scaffold.
Adds some tooltips to various demos.

Also some tweaks to stack.dart that I made before I decided not to go
down a "CustomPositioned" route.

Previously we used a positional argument for widgets that had multiple
children. Now we use a named argument that defaults to an empty list.

Fixes #241

Instead of taking a builder and a generic type, we can just take a
List<Widget>.