Add more fontification to regexp builder mode

Add more fontification to regexp builder mode

* lisp/emacs-lisp/re-builder.el (reb-copy): Work in the presence
of newlines in the regexps.
(reb-change-syntax): Use a dedicated history variable.
(reb-fontify-string-re): Fontify sub-matches.
(reb-regexp-grouping-backslash, reb-regexp-grouping-construct):
New faces.
(reb-string-font-lock-keywords): New variable.
(reb-mark-non-matching-parenthesis): Match parenthesis.
(reb-restart-font-lock): New function.

* lisp/emacs-lisp/re-builder.el (reb-mode-map): Add divider some
dividers (bug#6347).