bug#1305: All code that currently beeps should use visual bell instead

[Gregory Heytings]

> Or do we need a complete rethinking of this?  Perhaps there is some 
> other software that we can steal some good ideas from?

FWIW, what I would do for the default is to leave visible-bell to nil, and 
to use ring-bell-function to temporarily turn the cursor color to red and 
to put the message in the echo area on a yellow background.  Of course 
these colors would be configurable.  This would work on all platforms, and 
isn't too intrusive; at least it's less intrusive than the current 
defaults with visible-bell nil or t on GNU/Linux and macOS.