QQuickDwm: use QtWin::WindowFlip3DPolicy

Change-Id: Ifa74acbf7cde330838b37505e18a5b7376116216
Reviewed-by: Ivan Vizir <define-true-false@yandex.com>
Reviewed-by: Caroline Chao <caroline.chao@digia.com>
Fix QtWin namespace docs

Change-Id: I3d29698dd6e5f85691222f23817a8c3ac8d3447f
Reviewed-by: Caroline Chao <caroline.chao@digia.com>
Reviewed-by: Laszlo Papp <lpapp@kde.org>