Previously, we lost sync with the tab view contents when switching tabs. Now we
key the subtrees to make sure they keep their state across tab animations.

Fixes #3147

The TabBarSelection change animation needs to start where the fling's drag gesture ended rather than from zero.  The intial vlaue of progress for the TabBarSelection's performance is now converted from the range used during an interactive drag, to the range used when animating from the previously selected tab to the new one.

TabBarSelection now requires a maxIndex parameter.

The TabBar's selection is now represented by a TabBarSelection object which encapsulates both the previous and currently selected indices and the Performance used to animate the selection indicator.

Added a TabBarView class which displays a tab's contents. It uses a shared TabBarSelection to stay in sync with a TabBar. The TabBarView scrolls in sync with the TabBar when the selection changes. Eventually it will allow one to fling the selection forward or backwards.

Added a tabBar property to ToolBar. Typically the corresponding TabBarView will be the body of the toolbar's Scaffold.

Removed TabNavigatorView and TabNavigator.

Added a widget gallery tabs demo page. Removed the old tabs demo.

This package contains WidgetTester, which is very useful when writing tests for
widgets.

- force the time dilation to 1.0 for the Widget tests, so that a local
  change doesn't break all the tests during development.
- add missing license block to all the files.
- set ui.window.onBeginFrame to null when you use WidgetTester, so that
  the engine doesn't trigger any confusing frames after our fake frames.