bug#50766: Regexp paren pairs in query-replace-regexp prompt should be highlighted

[martin rudalics]

>> With 'blink-matching-paren' on do C-M-% and type [)].  People who use
>> 'blink-matching-paren' do not type complex regexps.
>
> We do indeed -- we just live with the bogus warnings from Emacs.

So a fix for both has to hook into post-self-insert and after-change.
Which means I have to prime these before-change to avoid parsing the
buffer twice.  Darn it!

martin