Refactor the callback part of an event into its own event_callback type This shouldn't have any visible effect, but it's necessary or advisible for a few changes and cleanups I would like to make, including: * Replacing the deferred queue with a type that works more as if it were an event. * Introducing a useful "activate this on the next round through the event loop" state for events and deferreds. * Adding an "on until further notice" status for events, to allow a saner win32-hybrid approach. * Eventually, making all user callbacks first-class things with event-like semantics.