Cook up some silly cases to test out the hidden-when='' attribute.
- make sure hidden-when='action-missing' shows/hides items based on
actions being created and destroyed
- make sure hidden-when='action-disabled' shows/hides items based on
actions being enabled and disabled
- make sure hidden-when='action-missing' doesn't hide items when the
action is merely disabled
https://bugzilla.gnome.org/show_bug.cgi?id=688421