After the refactoring of the animation library, we were missing some
dartdoc.

These should be the last references to the old "performance" name.

Some of the Animation classes that we converted from performances use
the term "master" to refer to the animation upon which they're based.
This patch changes them to use the term "parent", which is consistent
with the rest of the animation classes.

This patch removes Performance and AnimationValue now that we've ported the
framework over to AnimationController and Tween. This patch also cleans up the
names of the AnimationController classes now that they don't have to avoid
conflicts with the old animation API. Specifically, I've made the following
renames:

 * Animated -> Animation
 * Evaluatable -> Animatable
 * PerformanceStatus -> AnimationStatus

This patch is just renames and moving code around. There aren't any changes in
behavior.

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

This patch moves Navigator and related code over to using
AnimationController.