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 __init__() that is never removed.

This commit fixes the leaks by moving initialization of the observer callback to enter() and removing the callback in exit().

Fixes #4048