(hs-set-up-overlay): New user var.

(hs-set-up-overlay): New user var.
(hs-make-overlay): New function.
(hs-isearch-show-temporary): Handle `display' overlay prop specially.
(hs-flag-region): Delete function.
(hs-hide-comment-region): No longer use `hs-flag-region'.
Instead, use `hs-discard-overlays' and `hs-make-overlay'.
(hs-hide-block-at-point): Likewise.
(hs-hide-level-recursive): Use `hs-discard-overlays'.
(hs-hide-all, hs-show-all): Likewise.
(hs-show-block): Likewise.
Also, use overlay prop `hs-b-offset', not `hs-ofs'.