BUG: Fix memory leaks associated with py_StandaloneEditorWidgetTest
Created by: msmolens
In a debug build running running py_StandaloneEditorWidgetTest reports that a number of VTK class instances leak.
The leaks occur because EditorWidget adds an observer callback in
that is never removed.
This commit fixes the leaks by moving initialization of the observer callback to
enter() and removing the callback in