This should speed up the GUI (and the GUI tests massively). Closing the wizard calls the Close()
rather than Destroy() method, so the wizards are still in memory. A new call to an old GUI uf
object will simply call SetValue() for all keyword arg GUI elements, hence reset the element.
The Wizard GUI object has been modified to only add the buttons if they do not currently exist.
Without this, a new set of buttons is added each time the user function wizard appears.
