Also:
* Make PaginatedDataTable able to scroll itself horizontally.
* Make drop down buttons support having an explicit text style and icon
size given.
* Fix a bug with drop-down buttons asserting when opened partly
off-screen.
* Make sure to pop the drop-down button's route if the drop-down button
is discarded while the route is up.
* Remove extraneous padding on drop-down buttons. (Couldn't figure out
why it was there, and it breaks alignment when a drop-down is mixed
with other text.)
* Some docs improvements.
* Add Route.isActive
* Add a setState() method to ModalRoutes.
This introduces the key parts of a paginated data table, not including
the built-in pagination features.
* Provide more data for the data table demo, so there's data to page.
* Introduce a ChangeNotifier class which abstracts out
addListener/removeListener/notifyListeners. We might be able to use
this to simplify existing classes as well, though this patch doesn't
do that.
* Introduce DataTableSource, a delegate for getting data for data
tables. This will also be used by ScrollingDataTable in due course.
* Introduce PaginatedDataTable, a widget that wraps DataTable and only
shows N rows at a time, fed by a DataTableSource.
