Fix naming of abnormal hook vars in allout.el.

Fix naming of abnormal hook vars in allout.el.

* lisp/allout.el (allout-exposure-change-functions)
(allout-structure-added-functions)
(allout-structure-deleted-functions)
(allout-structure-shifted-functions): Rename abnormal hooks from *-hook,
and convert to defcustoms.
(allout-after-copy-or-kill-hook, allout-post-undo-hook): Convert
to defcustoms.
(allout-mode-hook, allout-mode-deactivate-hook): Doc fix.

* lisp/allout-widgets.el: Hook callers changed.