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.