Now that you import a top-level file for each layer of the Flutter
framework, within the framework we can use relative paths without being
worried about the copy/pasta problem we used to have.

This patch prepares us to create routes that control their performances more
closely.

Fixes #1170

This patch converts drawer to using the "openDialog" pattern for managing its
state. Currently, the drawer entrance and exit animation aren't integrated with
the navigator's animation system because the drawer's animations can be stopped
and reversed, which the navigator can't yet understand. That means dismissing
the drawer via the system back button causes the drawer to be removed
instanteously.

Fixes #715
Fixes #1187

This patch folds the functionality from AnimatedSimulation into Timeline.

Rather than having two objects driving scrolling animations, we now have one
object, a Timeline, drive both scrollTo and fling animations. Using Timeline
instead of AnimatedSimulation paves the way to removing AnimatedSimulation
(which is now used only inside the animation library).

Finally, this patch also simplifies (and makes private) _TweenSimulation by
using AnimatedValue to do the math.

AnimationPerformance had some logic for supporting multiple variables
that was hardly ever used. ValueAnimation, a subclass, has logic for
handling a single variable. I've removed the logic for handling
variables from AnimationPerformance in favour of most call sites instead
using ValueAnimation.

This fixes #1103.

- Reorder the methods so they are closer to run order.
- Reindent the code to match style guide.

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.

Used for AnimationPerformance with a single AnimatedValue<T> variable.

Just infer it from the sign of the velocity. That's what odeon does.

Also add intervals and curves to AnimationPerformance, which affect all
variables used by the performance.

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.

Simplifies the code a bit.

Use those in SnackBar, Drawer, navigator instead of AnimatedContainer's
intentions.

This required some changes to AnimationPerformance to better understand
animating with forces.

SnackBar, and PopupMenu instead of custom statuses.