QWidgetWindow stores the normal geometry obtained from the widget when
transiting to other states. This does not work reliably on Windows,
where this geometry is already that of the new state. Instead,
introduce  QPlatformWindow::normalGeometry(), add implementation
for Windows and use that in QWidgetWindow.

Task-number: QTBUG-21371

Change-Id: I3819ebaf55b4e7d2f7eef1affe6c20712ba45d7c
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>