SubmenuNavItem doesn't have a good solution for a navigation where the head of the submenu is itself clickable. A possible solution would be an additional field in SubmenuNavItem where another Node can be added, which is then rendered as the head of the submenu.

Also it might be good to prevent the placement of a submenu into this field, which would make no sense. Therefore introduction of groups for navigation items might be a good idea, analogous to groups in blocks/validation/etc.

Related Ticket: #1378 (For the field in the submenu formtype)