Engine Classvars

__singleton_key__: (classvar) reference key for 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