* calendar/todos.el: Further code rearrangement; further new and

* calendar/todos.el: Further code rearrangement; further new and
revised comments.
(todos-reevaluate-filelist-defcustoms): Rename from
todos-reevaluate-defcustoms and adjust callers.
(todos-date-pattern, todos-nondiary-start, todos-nondiary-end)
(todos-date-string-start, todos-done-string-start)
(todos-item-start): Change from defvar to defconst.
(todos-set-top-priorities): Use read-number and simplify.
(todos-insert-item): Check whether date-type argument is a string.
(todos-set-date-from-calendar): Check whether
todos-date-from-calendar is a string; simplify cond clause.
(todos-archive-done-item, todos-unarchive-items):
Use buffer-substring-no-properties.