* allout.el (allout-post-undo-hook): New allout outline-change event hook

* allout.el (allout-post-undo-hook): New allout outline-change event hook
to signal undo activity.
(allout-post-command-business): Run allout-post-undo-hook if an undo just
occurred.
(allout-after-copy-or-kill-hook), (allout-mode): Minor docstring changes.

* allout-widgets.el (allout-widgets-after-undo-function): Ensure the
integrity of the current item's decoration after it has been in the
vicinity of an undo.
(allout-widgets-mode): Include allout-widgets-after-undo-function on the
new allout-post-undo-hook.