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

[Stefan Monnier]

> I don't understand why flashing the mode-line is among the possible options.

As a happy user of the current GNU/Linux `visual-bell`, I don't really
care where the flashing happens, because I don't look at it.  I just
need some flashing somewhere within my field of vision as an indicator
that "something's going on": it doesn't need to be at the place where
I need to look.

And indeed, the current GNU/Linux visual-bell works quite well for that
in my experience since it flashes both above and below, thus not
mistakenly attracting the attention to a particular spot.
In other words, for me the ideal visual bell is one that I can "feel"
more than i can "see".

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.


        Stefan