For those times when you want to do something as you move away from a
route into the next one, as well as when you move into it from the
previous one.

This patch simplifies AnimationTiming and all the AnimatedValue base classes.
Also, make PopupMenu a stateless component because it has no state.

Fixes #1168

- Fix AnimationTiming to have defaults for 'interval' and 'curve' since
  that seems to be how we use it.

- Merge RouteBase.build and RouteBase.buildTransition

- Get rid of HistoryEntry, since it added nothing

- Broke out RouteBase.createPerformance() so subclasses can change what
  is created.

- Build the routes backwards so that we more efficiently avoid building
  hidden routes.

- Introduce an explicit way (!hasContent) for RouteState to avoid
  building, rather than the implicit "build returns null" we had before.

Surface all the constructor arguments of AnimationTiming in all its subclasses.
Remove some pointless casts.
Fix some typos.
Put constructors first in class declarations.
Remove some blank lines where they just confused the structure of the code.

Move the animation libraries into src/animation and change importers to use
package:sky/animation.dart. Also, move scheduler.dart into the animation
library so that the animation library can be self-contained.

This ensures we don't run into discontinuities when reversing an
animation halfway through. I refactored AnimationValue to have knowledge
of the reverse curves and intervals.