vtkSMProxyProperty and vtkSMInputProperty are closely related but due to
separate datastuctures for keeping track of state we had to  use
complicated logic. Cleaned that up.

This also fixes these properties to now fire modified events correctly
i.e. only when truly modified -- something needed to avoid duplicate
work in domains.

Change-Id: Ibe012970ccaca727643c2929f7dd3c0e46eb14a7