* calendar/todos.el: Further comment revision.

* calendar/todos.el: Further comment revision.
(todos-sorted-column): Change default value, also taking tty into
account.
(todos-reset-done-separator): Fix faulty variable binding.
(todos-reset-and-enable-done-separator): Save match data; comment
out code that causes problems for Edebug.
(todos-item-start): Handle empty line between todo and done items
when done items are hidden.
(todos-read-date): Use a leap year for `*' to allow
calendar-last-day-of-month to return Feb. 29.
(todos-archive-mode, todos-edit-mode, todos-categories-mode)
(todos-filtered-items-mode): Delete faulty parentheses.
(todos-quit): Save Todos and archive files unconditionally.
(todos-forward-item): Accept only positive prefix argument.
(todos-backward-item): Accept only positive prefix argument; don't
move point to beginning of buffer if it is on the first item.
(todos-hide-show-date-time): Remove obsolete interactive spec.
(todos-move-category): Improve prompt string; ensure file moved to
is different from file moved from.
(todos-merge-categories): Remove.
(todos-set-category-priority): New command.
(todos-raise-category-priority, todos-lower-category-priority):
Use it to define these commands.
(todos-set-item-priority): Rewrite and generalize.
(todos-raise-item-priority, todos-lower-item-priority): Use it to
define these commands.