qt5 didn't provide implementation of virtual SalMenu::ShowMenuBar
so native menu bar was visible at all times. Hopefully notebookbar
fans are happier now

Change-Id: Ie581397521369160e16f40231bade40100dcd6f6
Reviewed-on: https://gerrit.libreoffice.org/69327
Tested-by: Jenkins
Reviewed-by: Katarina Behrens <Katarina.Behrens@cib.de>