Primarily, this rearranges how listeners are handled internally by the
various PerformanceView classes so that there's not so much redundancy.

It also fixes ReversePerformance and ProxyPerformance to not leak.
Previously, they never unhooked their listeners, so they'd leak until
the entire chain of performances was killed. Now, they unhook as soon as
they themselves have no listeners left, which is more idiomatic.

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.