bug#69237: 30.0.50; Toggle password visibility

[Eli Zaretskii <eliz@gnu.org>]

> Date: Fri, 16 Feb 2024 14:33:02 +0100
> From:  Michael Albinus via "Bug reports for GNU Emacs,
>  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
> 
> We have `read-passwd', which reads a password in the minibuffer, hiding
> the input. However, modern password input boxes outside Emacs allow to
> show/hide the input while typing.
> 
> The appended patch tries to implement this feature. The minibuffer in
> `read-passwd' has bound TAB to a function, which toggles visibility of
> the typed input.
> 
> On graphic displays, the mode-line has an additional indicator for
> whowing/hiding the typed password. Clicking on this indicator with
> mouse-1 toggles the password visibility.

I'm not sure TAB is the best candidate for this.  I think it would be
better to use some non-character key, like F9 or somesuch.  And
another one, for keyboards without function key.

Also, this needs the appropriate documentation changes.

Thanks.