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.