Fixes #1170

This patch folds the functionality from AnimatedSimulation into Timeline.

Now that we've ported all the widget tests to fn3, we don't need a separate fn3
directory.

I've also removed the top-level description of the Sky package. Instead, we
should host that content on flutter.io.

This patch is part of a sequence of patches towards fewer top-level libraries.
In this patch, the gesture libraries are combined into one gestures.dart
library.

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.