Refactor the callback part of an event into its own event_callback type

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.