View source for 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 [https://github.com/Slicer/Slicer/blob/1c10718f30b8f15a5f6c0602e96add70c1898df6/Modules/Scripted/EditorLib/EditBox.py#L2 here]) but not in scripted loadable module classes.

We need to fix:
* the base class so that it derives from 'object' and be a [https://docs.python.org/2/reference/datamodel.html#newstyle "new style" python class]  and 
* the c++ proxy class so that we can understand new style classes. See [https://github.com/Slicer/Slicer/blob/master/Base/QTGUI/qSlicerScriptedLoadableModuleWidget.cxx qSlicerScriptedLoadableModuleWidget.cxx] and [http://stackoverflow.com/questions/18757186/pyinstance-newraw-with-old-and-new-style-classes pyinstance-newraw-with-old-and-new-style-classes]