Add view ID, rework assist API.

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