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