Documentation/Labs/PythonObserverCallbacks

Currently the syntax for defining observer callback functions with CallData argument is too comples, see Documentation/Nightly/Developers/FAQ/Python_Scripting#How_can_I_access_callData_argument_in_a_VTK_object_observer_callback_function

The remaining limitation is that mix-ins can only be used in helper classes (such as here) but not in scripted loadable module classes.

We need to fix:

• the base class so that it derives from 'object' and be a "new style" python class and
• the c++ proxy class so that we can understand new style classes. See qSlicerScriptedLoadableModuleWidget.cxx and pyinstance-newraw-with-old-and-new-style-classes