This is not a grand refactor yet, it's just cleaning up what we have
already, so that people who keep using this API (e.g. dialogs) have
something coherent to deal with.
The major changes are that Navigator and NavigatorState have the same
API now, that most of the examples use `<void>` instead of `<Null>`,
that the navigator observer can see replaces, and that the `settings`
is moved from ModalRoute to Route. I also cleaned up some of the API
documentation.
Since we can change the theme's platform, we need to make sure we
propagate that throughout rather than having half the app use the native
platform and half the app use the selected platform.
These futures complete when the route is popped off the navigator. This
generalizes and simplifies a mechanism already in place for dialogs and
menus.
Fixes#5283
