GtkMenuTracker: remove hidden items from the menu

GtkMenuTracker: remove hidden items from the menu

Modify the tracker so that it manages the visibility of
GtkMenuTrackerItem by issuing insert and remove callbacks to the
user of the tracker.

This works by treating the GtkMenuTrackerItem as a virtual section which
contains 1 item when the item is visible and 0 items when it is hidden.

For efficiency reasons, we only employ this trick in the case that the
item has a hidden-when='' attribute set on it.

https://bugzilla.gnome.org/show_bug.cgi?id=688421