ListStrEditor does not respect auto_add=True when using Qt

[itziakos]

When using the auto_add=True option an empty item is expected to be visible at the end of the list for the user to edit and append to the list trait that the ListStrEditor edits. However, setting auto_add=True does not look that it has any effect.

To replicate use the example
https://github.com/enthought/traitsui/blob/master/examples/demo/Advanced/ListStrEditor_demo.py

The problematic behaviour is present using qt (pyside) while it is working fine using wxpython