Add view ID, rework assist API.

Add view ID information to the assist structure.

Also rework the API to simplify how it works by removing
the ViewNode wrapper around ViewNodeImpl -- these are now
just the same thing.  And then add complexity by introducing
a formal WindowNode object that contains the top-level window
information (so I can add in some more window-specific info
in the future).

Change-Id: I5d525cf61ab6a73193e5cceb4c09d2d21cc27bae