'read-passwd' can toggle the visibility of passwords

'read-passwd' can toggle the visibility of passwords

* doc/lispref/minibuf.texi (Reading a Password):
* etc/NEWS: 'read-passwd' can toggle the visibility of passwords.

* etc/images/README: Mention the new images below.
* etc/images/conceal.pbm:
* etc/images/conceal.svg:
* etc/images/reveal.pbm:
* etc/images/reveal.svg: New images.

* lisp/simple.el (read-passwd--mode-line-buffer)
(read-passwd--mode-line-icon): New defvars.
(read-passwd--toggle-visibility, read-passwd-mode): New defuns.

* lisp/subr.el (read-passwd-map): Add 'TAB' binding.
(read-passwd--hide-password): New defvar.
(read-passwd--hide-password): Rename function from
`read-password--hide-password'.  Adapt callees.  Implement both
hiding and showing the password.  (Bug#69237)
(read-passwd): Call `read-passwd-mode'.