Improve interactive prompting for face colors

Improve interactive prompting for face colors

When displaying the completion candidates, show how the face would
look with the new foreground/background.

* lisp/faces.el (faces--string-with-color): New helper,
factored out from 'defined-colors-with-face-attributes'.
(defined-colors-with-face-attributes): Use it.
(read-color): Add optional argument FACE and pass
it to 'faces--string-with-color.'
(read-face-attribute): Call 'read-color' with more appropriate
foreground and face arguments.

* doc/lispref/minibuf.texi (High-Level Completion): Describe
the intention behind the arguments FOREGROUND and FACE of
'read-color'.  (Bug#64725)