Cleanup frame flushing.

Cleanup frame flushing.
* dispextern.h (struct redisplay_interface): Drop
flush_display_optional because flush_display is enough
for X and flushing via RIF is just a no-op for others.
* frame.h (flush_frame): New function.
* dispnew.c (update_frame):
* minibuf.c (read_minibuf):
* xdisp.c (echo_area_display, redisplay_preserve_echo_area):
Use it.
* keyboard.c (detect_input_pending_run_timers): Do not flush
all frames but selected one in redisplay_preserve_echo_area.
* nsterm.m (ns_flush): Remove no-op.
(ns_redisplay_interface): Adjust user.
* w32term.h (x_flush): Remove no-op.
(w32_redisplay_interface): Adjust user.
* xterm.c (x_flush): Simplify because we do not flush all
frames at once any more.  Adjust comment.
(x_redisplay_interface): Adjust user.