Scaffold appBar is-a PreferredSizeWidget, etc - #9380

Requiring a Scaffold's appBar to be an AppBar widget was at least inconvenient and definitely inconsistent with the overall Flutter API. Similarly the bottom of an AppBar had to be a widget that implemented AppBarBottomWidget; somewhat less of an obstacle but still odd.

These quirks in the API existed because sometimes it's useful for a widget to advertise its preferred default size without actually constraining its own size.

The PreferredSizeWidget interface can be used for these situations. AppBar and TabBar implement PreferredSizeWidget:

abstract class PreferredSizeWidget extends Widget {
  Size get preferredSize;
}

In both cases the returned size only limits the height, like new Size.fromHeight(100.0).

Scaffold's appBar and AppBar's bottom properties are now PreferredSizeWidgets. In the common cases where appBar is an AppBar and bottom is a TabBar, nothing needs to change.

To substitute an abitrary widget for PreferredSizeWidget, use PreferredSize. For example:

new Scaffold(
  appBar: new PreferredSize(
    preferredSize: const Size.fromHeight(200.0),
    child: const Text('HELLO WORLD'),
  ),
  body: ...
)

Scaffold now has a default-true primary property, like AppBar does. If true then the height of the [appBar] will be extended by the height of the screen's status bar. Apps that set the Scaffold's primary property to false will very likely also want to set the appBar's property to false.

The AppBar minExtent property has been replaced by preferredSize.

Fixes: #8970
Fixes: #9349