Fixed sit-for on gtk-screens using peek timeout.

Fixed sit-for on gtk-screens using peek timeout.

* src/edwin/input.scm (keyboard-peek-no-hang)
(handle-simple-events-until): Added optional "timeout" argument,
implemented with the new handle-simple-events-until procedure.  Pull
the simple event handling out of handle-simple-events' loop to create
simple-event-handled?, and used it to implement
handle-simple-events-until.

* src/edwin/simple.scm (sit-for): Get screens up-to-date first, even
with input pending(?).  Replaced busy loop with optional timeout
argument to keyboard-peek-no-hang.  This processes simple events,
which call update-screens! as necessary.