More flicker fixes for OSX, related to bug 18757.

* nsfns.m (ns_set_name_as_filename): Don't set represented filename
at once, call ns_set_represented_filename instead.

* nsterm.h: Declare ns_set_represented_filename.

* nsterm.m (represented_filename, represented_frame): New variables.
(ns_set_represented_filename): New function.
(sendEvent:): Set represented filename here to avoid flicker,
related to Bug#18757.