delivery of events to windows and clients.

This is tentative.  It's likely that an additional last-resort hook will
be necessary for code that calls TryClientEvents or WriteEventsToClient
directly.  It's also possible that new xace machinery will be necessary
to classify events and pull useful resource ID's out of them.

The failure case also needs some thinking through.  Should event delivery
"succeed" or should it report undeliverable?

Finally, XKB appears to call WriteToClient to pass events.  Sigh.