Cocoa: Don't update the menubar when popups are shown

Task-number: QTBUG-31532

Change-Id: I86084cb96bd1dd253b3e2e4413c06de053b95b3b
Reviewed-by: Romain Perier <romain.perier@gmail.com>
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>