* calendar/todos.el (todos-done-separator-string): New defcustom.

* calendar/todos.el (todos-done-separator-string): New defcustom.
(todos-done-separator): New variable replacing defcustom of the
same name.
(todos-reset-done-separator-string, todos-done-separator):
New functions.
(todos-mode): Make function added to
window-configuration-change-hook do a better job of updating the
done items separator string overlay.
(todos-unload-hook): Remove it here.
(todos-item-undo): Fix search for item's end.