patch 8.0.0683: visual bell flashes too quickly

patch 8.0.0683: visual bell flashes too quickly

Problem:    When using a visual bell there is no delay, causing the flash to
            be very short, possibly unnoticeable.  Also, the flash and the
            beep can lockup the UI when repeated often.
Solution:   Do the delay in Vim or flush the output before the delay. Limit the
            bell to once per half a second. (Ozaki Kiichi, closes #1789)