BUG: Fix memory leaks associated with py_StandaloneEditorWidgetTest

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 0004048

From: Max Smolens <max.smolens@kitware.com>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24562 3bd1e089-480b-0410-8dfb-8563597acbee