AppFlow, by @loganj et al[1]

Massive update to the sample up. Introduces AppFlow, which
may become a bunch of static methods on Flow itself. Allows
views to fish their Screen object out of the context.

Also ContainerView with massively configurable ScreenSwitcher,
and a basic FrameScreenSwitcherView. Includes long awaited
save and restore of view hierarchies.

Much documentation and polish needed.

[1] @dnkoutso, @rjrjr, @pyricau