Change camel-case header <QWinFunctions> to <QtWin>

Name the header after its namespace, to reduce the number of names
developers must remember.

Change-Id: I7b653886c1b504678d73117dfa4a9bfe2a2edbc4
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>
Reviewed-by: Morten Johan Sørvig <morten.sorvig@digia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>