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

[Stefan Kangas]

Lars Ingebrigtsen <larsi@gnus.org> writes:

> So perhaps we should instead discuss whether we should change the
> default of `ring-bell-function', and leave `visible-bell' as is?

I'm personally fine with that, but I suspect it would be controversial
as it would leave us with no way of beeping (visual or otherwise).