Unable to select directory in FileEditor on Windows

The `allow_dir` parameter for the `FileEditor` widget works for selecting a directory on macOS, but I could not get it to allow directory selection on Windows. Here's a minimal example based on the [`FileEditor` demo](https://github.com/enthought/traitsui/blob/master/traitsui/examples/demo/Standard_Editors/FileEditor_demo.py), with the addition of the directory parameter:

```Python
from traits.api import HasTraits, File
from traitsui.api import Item, Group, View, FileEditor

class FileEditorDemo(HasTraits):
file_name = File()
file_group = Group(
Item('file_name', style='simple', label='Simple', id='simple_file',
editor=FileEditor(allow_dir=True)),
)
traits_view = View(file_group)

demo = FileEditorDemo()
if __name__ == '__main__':
demo.configure_traits()
```

On macOS, I can select a folder and press Open, which closes the dialog and populates the text field with the path to the folder. On Windows, selecting a folder and pressing Open simply enters the folder. To select an item and close the dialog, I must select a file rather than a folder. A workaround is to select a file in the folder and then manually delete the file name from the path.

Tested on TraitsUI 7.2.1, Traits 6.2.0, PyFace 7.3.0, and PyQt5 5.15.4. I also went back to TraitsUI 6.0.0, Traits 5.0.0, PyFace 6.0.0, and PyQt5 5.12 but saw the same behavior.