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.
* controller, position and test
* Make controllers swappable
* WIP
* Create a ListWheelScrollPhysics
* Created picker and gallery demo and testing now
* Works. Ready to document and test.
* Document and add tests. Make the scroll controller more generic.
* minor cleanup
* review
* review
* fix tests
* stop using TransformLayers for now
