Improved the Comparative Viz panel making it easier to add parameter values for
explicit locations in the comparative view.

Improved vtkSMComparativeViewProxy to automatically update all the views each
time it renders.

Added mechanism to ServerManager to write Proxies that don't provide the
traditional properties API and yet participate in state save/restore and
undo/redo. Look at vtkSMComparativeAnimationCueProxy as an example.