AttributeError in list editor in Qt

On master, pulled this morning, I just got this traceback when I changed the content of an entry in a list:
```
Traceback (most recent call last):
File "/Users/jrocher/.edm/envs/dev_py27/lib/python2.7/site-packages/traits/trait_notifiers.py", line 519, in _dispatch_change_event
self.dispatch( handler, *args )
File "/Users/jrocher/.edm/envs/dev_py27/lib/python2.7/site-packages/traits/trait_notifiers.py", line 614, in dispatch
handler( *args )
File "/Users/jrocher/Projects/ETS_source/traitsui/traitsui/qt4/list_editor.py", line 251, in update_editor_item
proxy = control.proxy
AttributeError: 'QPushButton' object has no attribute 'proxy'
```

I will make a standalone script to show the problem, but thought I would report quickly since this editor was just worked on...

Seen on PyQt 4.11.4-7, Master TraitsUI, and master Pyface, Python 2.7.
cc @corranwebster since you were working on the `ListEditor` this past day.