Use a callback object instead of ad-hoc lists for sync and frame events

So obvious in retrospect.  The object system can do all the work for us
and keep track of pending calls as regular objects and we don't need to
abuse the resource system to get them cleaned up on client exit.  We
don't need the custom key management or (broken) lookup, we just sue
object IDs.  And last but not least, anybody can receive the callback,
not just display listeners.