Make QtWin a Q_GADGET to expose the enums to the meta-system

This allows us to use the QtWin enums in QML.

Change-Id: I62cd21d850a9c2222ce901f166ab99859164a7fd
Reviewed-by: Ivan Vizir <define-true-false@yandex.com>
Reviewed-by: Caroline Chao <caroline.chao@digia.com>
Reviewed-by: Laszlo Papp <lpapp@kde.org>
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>