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

[Gregory Heytings]

> I think, perhaps, introducing a new visible bell (across all significant 
> system), that's considerably less intrusive than the ones we have today 
> on GNU/Linux and Macos, might be an option.  And then defaulting to 
> using that.
>
> Gregory suggested blinking the cursor in a different colour -- perhaps?

I attach a POC of what I have in mind.

visible-bell.el
Description: Text document