Introduce Windows version 8.1 and detect by checking the version.

Change-Id: I04012218c34f7a95a87fcf2dc7fc095f0e743e67
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
Reviewed-by: Oliver Wolff <oliver.wolff@digia.com>
Reviewed-by: Andrew Knight <andrew.knight@digia.com>
Reviewed-by: Björn Breitmeyer <bjoern.breitmeyer@kdab.com>
(cherry picked from qtbase/dff6d73d6c37c40790d90bfdf3cb2f6e75c5bfa4)