GtkMenuTrackerItem: fix submenu visibility flag

GtkMenuTrackerItem: fix submenu visibility flag

We were only properly setting the "is-visible" flag to TRUE for menu
items with associated actions and not (for example) on submenus.

This was fine because the code for building GtkMenus from models
(correctly) assumed that submenus should always be visible and never
checked the property.

This is not true for the Mac OS code, which actually checked the
property and found it to be false for submenus.

Initialise the property to TRUE so that we get the correct value
reported for items that don't have actions.

https://bugzilla.gnome.org/show_bug.cgi?id=735122