This attempts to apply some of what we learnt from UX studies, namely
that people wonder how to add multiple children to widget that take
one child.

...and break the navigator<->routes circular dependency.

This is a step towards fixing https://github.com/flutter/flutter/issues/9577 but doesn't yet do so.

* more widget const constructors

* prefer const constructors

* address review comments

There are more places we can use this annotation, but this patch just gets us
started.

Rename State.config to State.widget
Rename State.didUpdateConfig to State.didUpdateWidget
Renamed all State subclasses' local variables named config to something else

The new name matches the style guide. (The old name was just old and predated
the style guide.)

Fixes #8000

This functionality is independently useful.

Fixes #8293