* calendar/todos.el: Improve handling of overlays.

* calendar/todos.el: Improve handling of overlays.
(todos-get-overlay): New function.
(todos-prefix-overlay): Remove, since subsumed by
todos-get-overlay, and replace by the latter in callers.
(todos-reset-prefix): Apply only to buffer visiting Todos files.
Simplify implementation and use `todos' overlay property.
(todos-reset-done-separator): Use todos-get-overlay and `todos'
overlay property.  Fix logic.
(todos-category-select): Use todos-get-overlay and `todos' overlay
property.
(todos-remove-item): Use todos-get-overlay.  Correct obsolete code.
(todos-prefix-overlays): Use todos-top-priority face also for
non-numerical prefix of top priority items.  Add `todos' overlay
property.
(todos-hide-show-date-time): Simplify, using todos-get-overlay and
`todos' overlay property.