Sample app shows master /detail in tablet landscape.

Some Flow API tweaks in support of that, and just general cleanup.
In particular, most reset methods take direction param. Still
not sure what to do with resetBackTo though, which emits either
BACKWARD or FORWARD.

The app is ugly. In particular its master view needs to be
highlighting the current detail view, and I haven't touched the
action bar at all. But The actual code to split the screens
is so, so lovely.