Menu: Support invisible MenuItems

Note that currently, invisible Menus in MenuBars are not supported
because of the QPA backend. To be decided whether we want this...

Change-Id: Ide01c0ddc12f07969a5466a77b5c5c67b265ec87
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>