quartz menu: update visibility property name

This property is called "is-visible" now, not "visible".

https://bugzilla.gnome.org/show_bug.cgi?id=735122
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
GtkMenuTracker: one more visibility tweak

On creation, we call action_removed() in case the action was missing
from the start.  Because we just created the action, 'can_activate' will
always be FALSE here and this function will therefore always do nothing.

We do want the visibility state to be updated though, for the case where
the action is missing but the item should still be visible from the
start.

Update the visibility directly instead of trying to call
action_removed().

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