First pass on sprite 3D node

Fixes path in SerializePicture call

Optimizes drawing performance in sprites

Enable GrLayerHoister optimizations in Skia

Turns out we weren't passing kComputeSaveLayerInfo_RecordFlag, which meant we
weren't computing the information necessary for the GrLayerHoister in Skia to
do its work.

This CL makes demo_launcher scrolling 3x faster on a Nexus 5.

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

Refactors the sprite matrix handling to do most calculations in Dart

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

This file was deleted.

Avoid crashing when building an app without an app.skyx file in DEBUG mode.

app.skyx file in DEBUG mode.

Fix failing test card_collection.dart

Add a widgets.dart uber library

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

Rename some animation classes.

* Intentions => Behaviors
* SlideIn => SlideTransition
* FadeIn => FadeTransition
* removed unused AnimatedContainer intentions

Use transitions for Dismissable

Use SkImage instead of SkBitmap

Fix visual bugs in Stocks

Apparently SkImage is the new hotness.

Simplifies the code a bit.

Add support for FontStyle.italics

 - Center the labels on the tabs
 - Make the drawer's mask visible

Allow iOS apps to restart by copying the app.skyx to the Documents directory on the device.

directory on the device.

This work is to support a workflow where developers testing on an iOS
device will automatically see their app updates as they change their Sky
code. Currently this works by using the ios-deploy tool:
$ ios-deploy --bundle_id 'org.domokit.sky.game' --upload out/ios_Debug/game_app.app/app.skyx --to Documents/app.skyx
Upcoming commits will incorporate this into skytool.

Fix skyanalyzer