bug#6227: Color isearch regexp submatches differently

[Drew Adams <drew.adams@oracle.com>]

> >   (defface isearch
> >     '((((class color) (min-colors 88) (background light))
> >        ;; The background must not be too dark, for that means
> >        ;; the character is hard to see when the cursor is there.
> >        (:background "magenta3" :foreground "lightskyblue1"))
> 
> I meant that these deffaces aren't complete -- they look like examples,
> and the examples are taken from the isearch face.
> 
> There should be more of them, and they should be complete, and they
> should have better names (isearch-group-1 etc perhaps).

FWIW, I use simple definitions like this one for the
eight regexp-level faces, not bothering with multiple
kinds of displays.  (Not saying that's what isearch.el
should do.)

(defface isearchp-regexp-level-8 
  '((((background dark)) (:background "#12EC00003F0E")) ; a very dark blue
    (t (:background "#F6F5FFFFE1E1")))  ; a light yellow
  "Face used to highlight subgroup level 8 of your search context.
This highlighting is done during regexp searching whenever
`isearchp-highlight-regexp-group-levels-flag' is non-nil."
  :group 'isearch-plus :group 'faces)