(isearch-hide-immediately): New variable.

(isearch-hide-immediately): New variable.
(isearch-close-unecessary-overlays): New function.
(isearch-range-invisible): Use them.

(search-invisible): Changed the semantics,
the default value and updated the doc string.
(isearch-opened-overlays): New variable.
(isearch-mode): Initialize it.
(isearch-switch-frame-handler): Call isearch-clean-overlays.
(isearch-exit, isearch-cancel, isearch-abort): Likewise.
(isearch-other-meta-char, isearch-search):
Support the new meaning for search-invisible.
(isearch-open-necessary-overlays, isearch-open-overlay-temporary)
(isearch-clean-overlays): New functions.
(isearch-range-invisible): If we are inside overlays that can be
opened, open them, add them to isearch-opened-overlays and say
that the range is visible.