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

[Gregory Heytings]

> > My point was that we can clean up without any new variable ;-)
>
> I'm not sure I follow you at all here.  :-/

Stefan M means (IIUC) that, instead of introducing a new variable to clean 
up the current situation, it is better to reuse the already existing 
ring-bell-function variable, to give it some new possible values, and at 
the same time to deprecate the visible-bell variable.