* calendar/todos.el: Display numerical priority string of top

* calendar/todos.el: Display numerical priority string of top
priority items in category in a distinctive face.
(todos-prefix-string): Make doc string more precise.
(todos-top-priority): New defface.
(todos-done, todos-done-sep): Use more compact face definition
\(taken from font-lock.el).
(todos-comment): Give a complete face definition, instead of
inheriting from todos-done.
(todos-font-lock-keywords): Use todos-comment-face for
todos-comment-string-matcher.
(todos-prefix-overlays): Use todos-top-priority as the face
property of top priority items; don't condition reapplying item
prefix overlay on whether the string changed, since that prevents
updating display after changing number of top priorities.
(todos-set-top-priorities): Call todos-prefix-overlays to update
display.