existing event calls to use state as "offline" or "online", with "change"
indicating the reason for this status event. Changed disconnect() to send
state "offline", change "disconnect" -- instead of replaying last auth
failure. Changed unit tests accordingly.
Changed LLLoginInstance::handleLoginEvent() to use LLEventDispatcher to route
calls to handleLoginFailure() et al.
Added LLEventDispatcher::get() to allow retrieving Callable by name and
testing for empty().