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

[Gregory Heytings <gregory@heytings.org>]

>> "Flashing a window means changing the appearance of its caption bar as 
>> if the window were changing from inactive to active status, or vice 
>> versa. [...] Typically, a window is flashed to inform the user that the 
>> window requires attention but that it does not currently have the 
>> keyboard focus." [1]
>>
>> [1] https://docs.microsoft.com/en-us/windows/win32/api/winuser/nf-winuser-flashwindow
>>
>> But even that doesn't describe the effect accurately, the feeling I 
>> have is closer to "the frame briefly vibrates".  As I said, I use 
>> Windows only rarely, so my feeling is perhaps not right.
>
> What I see is exactly what the MS documentation describes: the frame's 
> title bar blinks (a.k.a. "flickers"), which is very hard to notice; and 
> the corresponding tab on the task bar flashes, which is much more 
> prominent.  IME, this happens in all Windows versions.
>

What I see on Windows 10 (without any customizations) is that the title 
bar flickers, the menu bar flickers, and the shadow effect around the 
frame flickers.  All this gives the feeling that the frame "vibrates".

And yes, the corresponding tab (on Windows 10 the default is an icon) 
flashes, its background color changes.  Indeed this is more prominent, the 
purpose of FlashWindow seems to be, as the documentation describes, "to 
inform the user that the window requires attention but that it does not 
currently have the keyboard focus", or IOW to inform the user that a 
window that it isn't the foreground window requires attention.