bug#81583: 32.0.50; Optional preview of the replacement text while typing it

> Here's an almost final scratch of what I'm working on.

Thanks for the patch.  I have tested it, and it makes UI nicer.

> Before I go ahead with tests and manual entries, I'd like to share it
> with you for reviewing code, feature expectations, grammar, doc length
> and so on.

I have a comment about doc length: the amount of text
in the paragraph explaining the details of the
function of two arguments looks disproportionally long.

> Users can also set 'query-replace-preview-separator' to:
>
> - nil (automatic default, will use → or -> if not available)
> - ""  (a custom string in case of wanting another symbol or adding spaces
>        around, like " → ")

Not sure if we need a special option
'query-replace-preview-separator-string'
since in rare cases it's possible to
change the string using the function in
'query-replace-show-preview'.  (Or at least
postpone addition until requested by users.)

Also the face 'query-replace-preview-separator'
doesn't look necessary now since it can be added
by the same function (a face still could be added
later when requested by users).

> Regarding faces, following this scheme:
>
> [match]→[replace]
>    1   2    3
>
> The provided faces are:
>
> 1 -> query-replace-preview-match
> 2 -> query-replace-preview-separator
> 3 -> query-replace-preview
>
> Please find attached the diff and some screenshots.

Since you added dedicated faces, I have a better suggestion
for their default colors.  Like the result of replacements
can be viewed as a diff (there is even a key 'd' that shows
a diff during replacement), let's use diff colors by default
where the match has the red background, and the replacement
the green background, i.e. colors from diff-mode faces
'diff-removed' and 'diff-added'.  Like on the screenshot
Karthik posted to emacs-devel.  This removes the need
to use strike-through that makes the text less readable.