Set correct parent menu in function Qt5Menu::InsertMenuItem
instead of just returning it and setting it later if needed.

Change-Id: Id866987e91ec1d57c720735da6afc44109aa901a
Reviewed-on: https://gerrit.libreoffice.org/68437
Tested-by: Jenkins
Reviewed-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>