ENH:  Integrate Python testing framework with ParaView.  If testing with python
      is enabled, the play/record menus/dialogs give python as an option.

      It currently can query any QObject property and in python you can compare
      it to a known value.  Just throw an exception if the test fails.
      It can also take snapshots of arbitrary widgets and compare them to
      baselines.

      Also added a test that loads a histogram and takes a snapshot of the
      histogram widget and compares it to a baseline.