Notes on QDialog native dialog implementation

QDialog allows for platform helpers. This seems to be implemented by a few hooks that are based on nativeDialogInUse. But all the machinery for showing the widget is still run, but with WA_DontShowOnScreen set for the top level widget.

That means we're both creating a native window for the top level, and doing all of the show-handling of all the child widgets of the non-native fallback, just to conclude in the top level show_sys that we won't show it on screen after all.