mdi_application_window.py example problems

https://github.com/enthought/pyface/blob/master/examples/mdi_application_window.py

When running the example mdi_application_window.py on qt I get:

```
(qt-env-pyface)~/Desktop/pyface/examples aayres (master) $ python mdi_application_window.py
Traceback (most recent call last):
File "mdi_application_window.py", line 57, in
window = MainWindow()
File "mdi_application_window.py", line 35, in __init__
super(MainWindow, self).__init__(**traits)
File "/Users/aayres/.edm/envs/qt-env-pyface/lib/python3.6/site-packages/pyface/base_toolkit.py", line 184, in __init__
raise NotImplementedError(msg % (toolkit, package, name))
NotImplementedError: the qt4 pyface.ui.qt4 backend doesn't implement mdi_application_window:MDIApplicationWindow
```

and on wx:

```
(wx-env-pyface)~/Desktop/pyface/examples aayres (master) $ python mdi_application_window.py
02:08:25 PM: Debug: Adding duplicate image handler for 'Windows bitmap file'
02:08:25 PM: Debug: Adding duplicate image handler for 'Windows bitmap file'
02:08:26 PM: Debug: Adding duplicate image handler for 'Windows bitmap file'
/Users/aayres/.edm/envs/wx-env-pyface/lib/python3.6/site-packages/wx/lib/agw/aui/framemanager.py:4742: UserWarning: A pane with the name 'mdiclient' already exists in the manager!
warnings.warn("A pane with the name '%s' already exists in the manager!"%pane_info.name)
```

here is a screenshot of the resulting window (only appears on wx):