Demo application crashes if run with pythonw on Windows

To reproduce, with the current master and on Windows:
```
pythonw traitsui\examples\demo\demo.py
```
Then click on a file node on the tree. There will be a dialog saying "Python has stopped working".

After #1023 goes in, it can be reproduced with a different command (and with traitsui source installed to get some data):
```
pythonw -c "from etsdemo.main import main; main()"
```

For now, we can workaround this by not using `gui_scripts` for the demo main function entry point.