error on gaphor exit

Currently the problem is limited to errors in the namespace model.

This may be solved by unregistering the handlers properly on destruction.

Traceback (most recent call last):
  File ".\gaphor\action.py", line 152, in _action_activate
    method()
  File ".\gaphor\services\filemanager.py", line 139, in new
    element_factory.flush()
  File ".\gaphor\UML\elementfactory.py", line 143, in flush
    value.unlink()
  File "C:\workspace\python\gaphor\gaphor\UML\element.py", line 81, in unlink
    prop.unlink(self)
  File ".\gaphor\UML\properties.py", line 388, in unlink
    value.unlink()
  File "C:\workspace\python\gaphor\gaphor\UML\element.py", line 78, in unlink
    component.handle(ElementDeleteEvent(self._factory, self))
  File "c:\documents and settings\arjan\home\py-site-packages\zope.component-3.4
dev_r72749-py2.4.egg\zope\component\_api.py", line 135, in handle
    for ignored in sitemanager.subscribers(objects, None):
  File "c:\documents and settings\arjan\home\py-site-packages\zope.component-3.4
dev_r72749-py2.4.egg\zope\component\registry.py", line 290, in subscribers
    return self.adapters.subscribers(objects, provided)
  File "c:\documents and settings\arjan\home\py-site-packages\zope.interface-3.4
.0-py2.4-win32.egg\zope\interface\adapter.py", line 535, in subscribers
    subscription(*objects)
  File ".\gaphor\ui\namespace.py", line 183, in _on_element_delete
    path = self.path_from_element(element)
  File ".\gaphor\ui\namespace.py", line 76, in path_from_element
    n = self._nodes[ns]
KeyError: <gaphor.UML.uml2.Class object at 0x01BA6450>

Last issue was already reported in #65.