* calendar/todos.el: Improve handling of saved top priorities items.

* calendar/todos.el: Improve handling of saved top priorities items.
(todos-find-item): Find done items and items whose priority has
changed or whose text was truncated or augmented; add doc string.
(todos-check-top-priorities): Highlight each item that is not up
to date.
(todos-jump-to-item): Jump to items whose priority has changed or
whose text was truncated or augmented.