... and use them to improve the horizontal_scrolling example.

Now ScrollableBlock can combine a horizontally scrolling viewport with a
horizontal block.

Also rename ViewportScrollDirection to just ScrollDirection for less verbosity.

Teach Block how to layout horizontally

We'll need this for horizontally scrolling lists.

Add Fitness.apk with icon

This is just one of many steps in preparation for turning
fitness into a play-store ready app.

@abarth @collinjackson

Add a few bits of documentation to package:sky

restart Sky apps on Android without having to restart SkyShell.

Add a basic radial reaction to the switch

We still need to tune the various parameters, but this patch is a start.

workaround for Fitness demo dialog cancel bug

Adds cull rect to sprite performance test

I can't really make further progress until at least:
https://github.com/dart-lang/dartdoc/issues/766
is resolved, ideally also:
https://github.com/dart-lang/dartdoc/issues/763

@abarth

Initial version of game performance tests

Fix RenderViewport's hit testing

We were subtracting the scrollOffset when we were supposed to add it.

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.

It was confusing to have both widget.dart and widgets.dart

Make sky_tool much more user-friendly.

This refactors Checkbox to own a RenderObject similar to how Switch was
refactored in https://github.com/domokit/sky_engine/pull/376 and
extracts common functionality for toggleable renderers into a base
class.  Switch and Checkbox's render objects derive from this base
class to add their own custom painting and theming logic.

B=Fixes #187

Teach event system about disposition and make 'consumed' disposition …

This introduces the notion of event disposition and allows event
targets (widgets and render objects) to consume events that should not
be processed further. This is needed by the Switch component in the
Drawer in the stocks example. The Switch is embedded in a DrawerItem.
The Switch handles the gesture tap event to toggle its state and should
handle pointer events to allow swiping and draw its own radial
reaction. The DrawerItem also handles gesture taps to allow toggling
the switch value when tapping anywhere on the drawer and to draw its
own ink splash. When tapping on the switch, both the switch's render
object and the DrawerItem's listener are in the event dispatch path.
The Switch needs to signal in some fashion that it consumed the event
so the DrawerItem does not also try to toggle the switch's state.

Refactors the sprite matrix handling to do most calculations in Dart, minimizes the number of calls down to Skia

Refactors matrix handling in sprites