AppBar extensions for Scaffold et al
Add support for the appbar behavior described in the "Flexible space with image" section of https://www.google.com/design/spec/patterns/scrolling-techniques.html#scrolling-techniques-scrolling.
For now, an "AppBar" is a toolbar, a toolbar and a tabbar arranged in a column, or a toolbar and flexible space widget arranged in a stack with the toolbar on top. The AppBar occupies the space behind the status bar, i.e. its origin is at the top of the screen.
The Scaffold listens for scroll notifications from a Scrollable descendant identified by scrollableKey. Scrolling downwards from the top causes the Scaffold to shrink its appBar's height from appBarHeight to 0. Scrolling downwards when the appBar isn't visible causes its toolbar to reappear. Applications can specify the flexibleSpace - essentially the appbar's background - with a WidgetBuilder. The contents of the flexible space can track the appbar's relative height with Scaffold.of(context).appBarAnimation, an animation that's 0.0 when the appbar is fully visible and 1.0 when it has disappeared.
Added FlexibleSpaceBar to simplify building a flexible space widget that animates a title and a background image per the Material Design spec.
To enable the new behavior, specify the following Scaffold properties:
```
return new Scaffold(
appBarHeight: appBarHeight,
scrollableKey: scrollableKey,
appBarBehavior: AppBarBehavior.scroll,
...
)
```
AppBarHeight should be greater than kToolBarHeight. Typically it will be much greater.
ScrollableKey identifies the Scrollable that the Scaffold will track.
AppBarBehavior.scroll indicates that the appbar will resize in response to scrolling.
The Scrollable must be a descendant of the Scaffold body and must specify the same scrollableKey as well as a padding value that accounts for the appbar's height. For example:
```
body: new ScrollableViewport(
key: scrollableKey,
child: new Padding(padding: new EdgeDims.only(top: appBarHeight)
...
)
```
Also:
- Added scrollableKey and scrollablePadding to MaterialList. They're applied to the ScrollableList child.
- Fixed the padding support in ScrollableList.
- Added a foregroundAlpha property to ToolBar. It controls the opacity of the icon and text themes' colors.
- Removed the toolbar's "bottom" widget property. Use the flexibleSpace WidgetBuilder instead.
We were getting confused about our layer tree offsets, which caused us
to set an incorrect cull rect for the recording inside a viewport.
This patch does away with layer tree offsets almost entirely. We now use
them only at repaint boundaries, which is where we want the layer tree
to be mutable.
Fixes#1460
Now the text input control knows how to move the caret when you tap
inside the string. There's still some rough edges to polish up, but this
patch is the first step.
Fixes#108
This patch teaches the editing system to paint reasonable selections for
single-line text fields, including for bidirectional text.
Requires https://github.com/flutter/engine/pull/2318
