We've decided not to store the scroll position in PageStorage because
routes now maintainState by default.

Fixes #8051

This patch improves PageView to the point where we can use it in the date
picker. Specifically, you now get onPageChanged notifications and you can
control which page is visible using a PageController.

There aren't any interdependencies with the rest of sliver.dart, and that file
is already quite complex.