- remove Engine's new_user_object_class callback, not needed in 2.0
- remove Engine's update_user_objects callback, not needed in 2.0
- remove Engine's user_object_changed callback, not needed in 2.0
- remove Engine crash callback, unused in 2.0
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- remove unused instance variable @num_known_user_object_classes
- move USER_OBJECT_EXCEPTION_FORMAT out of engine to where it's used
- remove duplicate declaration of BASE_SET_PATH
- add ProjectEffect#pretick! for consistency with tick!
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- comment