Engine Classvars
----------------

.. glossary::

   __singleton_key__
     (classvar) reference key for :class:`Singleton` instances.
     any class with an identical key will be treated as an instance of that type.

   __destroyable__
     classvar indicating whether instances of this class (or subclasses who do not
     override this value) can be destroyed at runtime

   __exports__
     the name a script will be registered as into registry

   __create_listener_list__
     doc later

   __create_event_listeners__
     doc later