MaterialApp assumes that you're using material design.

Also move radial reaction and fix imports for stats box.

These are now part of material.dart.

Now SnackBar is an ephemeral route that uses a Placeholder to put itself into
the Scaffold.

Fixes #673

(These are changes cherry-picked from in-flight branches since they are
more independent and could be helpful even without those changes.)

- Change RouteBuilder's signature to take a single argument in which the
  other fields are placed, so that we can keep iterating on those
  arguments without having to break compatibility each time. Also, this
  makes defining route builders much simpler (only one argument to
  ignore rather than a variable number).

- Expose the next performance to RouteBuilders, since sometimes the
  route itself might not be where it's used.

- Allow BuildContext to be used to walk children, just like it can for
  ancestors

- Allow BuildContext to be used to get the Widget of the current
  BuildContext

- Allow StatefulComponentElement to be referenced with a type
  specialisation so that you don't have to cast when you know what the
  type you're dealing with actually is.

Also, introduce Colors and Typography to hold the material colors and the
typography declarations. Previously we expected clients of these libraries to
import them into a namespace, but that doesn't play nice with re-exporting them
from material.dart.

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.

The editing directory just defined two widgets. We might as well fold them into
the main widgets library.

I'm not sure this is the correct approach, but it
at least mitigates https://github.com/domokit/sky_engine/issues/804
for now.

I also made it not crash if you haven't set a goalWeight.

@collinjackson

Fixes #743

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

This required me wrapping the new Activity APIs
as well as adding the missing pubspec.yaml

@abarth

Now you can import 'package:sky/widgets.dart' to get all the widgets.

We ended up with a bunch of different scrollable widgets and it was confusing.
This CL moves them all into one library to make them easier to find.

Converted demo launcher example to library

Converted fitness example to library

Converted hello world example to library

Converted mine digger example to library

Converted stocks example to library