Set more window flags to keep Windows from omitting titlebar & buttons

This should not be necessary, but Windows does not treat the missing
Qt::CustomizeWindowHint the same as the other platforms do.

Task-number: QTBUG-35049
Change-Id: I81dc645dc1acf29b6ff4d5fc04f5dd3db94b0133
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>