Adds rough sound support for sprites

Rename createsNewDisplayList to requiresCompositing

This patch prepares for introducing the compositing system by repurposing the
relevant parts of the createsNewDisplayList system. This patch also removes the
no-longer-relevant parts of that system.

Fixes issue in sprite

We seem to have trouble killing sky_server by its pid. Instead, lets try
killing it by looking up which process is listening on the sky_server port.

Fix common asserts in animation API.

We'll need this for compositing because we need to switch out the sky.Canvas
when we switch compositing layers.

1. We would assert if you tried to start an animation from within an
animation status callback. This is a common pattern, so I fixed this
assert (in Ticker._tick).
2. We would assert for animations with duration under a millisecond.
Fixed.

Also removed the workarounds assocated with #1.

Previously we were doing the bounds testing for hit tests in the parent, but
that doesn't work if the child paints at a location other than 0.0, 0.0. Now we
do the bounds check in the child. This also simplifies Scaffold's hit testing.

Fixes #558

Add support for charts to fitness app

Adds ensureWidgetIsVisible() function to scrollable.dart

Maximum bounds for the mimic in the overlay can now be specified.

Adds API docs to particle systems and removes redundant property

Set the scrollOffset of a widget's Scrollable ancestor so that the
widget is centered within the scrollable. A future CL will add
support for specifying exactly where the widget appears. The scroll
can be animated by specifying the animation: parameter.

Changed the duration Scrollable.scrollTo() parameter from a Duration
to an AnimationPerformance so that one can configure all aspects of
the animation. The caller may also listen to the animation to schedule
other work while it updates or when its status changes.
complete

This is just fallout from not having
https://github.com/domokit/sky_engine/issues/565

@abarth

@abarth

@abarth

Attempt to make our splashes closer to other Android apps

And also Material Design of course.

@abarth

Don't paint twice

In refactoring the way we generate the SkPicture, I accidentially made us
generate the SkPicture twice. This CL causes us to generate it only once.

Adds API docs to sprite Node and Node3D

This introduces the concept of an Anchor, which you can use to link
transitions together. I've used this in the Fitness and Stocks apps to
link the FAB and SnackBar to animate together by sharing the
SlideTransition.

I also fixed the Scaffold hit testing code to apply sub-widget
transforms, so it works with Transformed nodes.

Adds API docs to sprite ColorSequence and Layer

Make Fitness tiles nicer

Adds documentation for sprite actions

Moved from Cards to Tiles and made the printing
of the dates nicer by using some code from a Dart SDK example:
https://github.com/dart-lang/sdk/blob/master/samples-dev/swarm/swarm_ui_lib/util/DateUtils.dart

I also built a UserData class to help keep saving/sorting
consistent as well as fixed the sort order to have most
recent at the top.

@abarth