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

[Gregory Heytings]

> I proposed the mode-line as an alternative for those who don't like the 
> current GNU/Linux flash, because it's one of the very few elements which 
> are displayed in almost 100% of the cases.  We could flash the echoarea 
> instead, but I think it's a bit more delicate to implement and it could 
> get in the way of reading the actual error message.

The good news is that it has been implemented ;-)