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

[Gregory Heytings]

> In practice, I like the current GNU/Linux's default better than your 
> proposal. We could work on converging on some version that satisfies 
> all, of course.

I know you like the current default on GNU/Linux, and it will remain 
available.  And what you like is not the default (you need to set 
visible-bell in your init file), so from that point of view nothing 
changes.

> > Would you imagine such a behavior in Visual Studio, Sublime or Atom?
>
> Briefly flashing some UI elements in a neutral fashion, without extra 
> colors that may look out of place?

Using inverse-video also creates extra colors, unless your frame happens 
to display only black on white or white on black elements on the first and 
last line.  Moreover with my patch the colors are fully configurable, so 
you can adapt them to your theme.

> That's also why I asked whether somebody knows a corresponding UI 
> element/animation in either of these editors we could, uh, "get inspired 
> by".

AFAICS in other editors error signals are far less frequent (e.g. they do 
nothing when you try to move past the beginning or end of the buffer, or 
when you press a key binding with no corresponding action, or when you 
enter characters in a read-only file, ...), they only signal "critical" 
errors.  So I'm not sure it's possible to get inspired by what they do. 
What they use are typically popups; I attach two examples with Visual 
Studio and Atom, one when a non-readable file is opened, another when a 
non-writable file is saved.

atom-open.png
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atom-save.png
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vscode-open.png
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vscode-save.png
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