DEV-40930: Added ["change"] key to login-module status events. Changed

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().