Support new-style classes for scripted modules/widgets; use VTKObservationMixin in EditorWidget
Created by: msmolens
This change makes EditorWidget use VTKObservationMixin to simplify managing observers, as suggested in https://github.com/Slicer/Slicer/pull/346#issuecomment-141474443.
To support using VTKObservationMixin with EditorWidget, this change also adds support for writing scripted module and widget classes as new-style Python classes.