* calendar/todos.el: Further significant code rearrangement;

* calendar/todos.el: Further significant code rearrangement;
further comment revision.
(todos-mode-display): New defgroup.
(todos-prefix, todos-number-priorities)
(todos-done-separator-string, todos-done-string)
(todos-comment-string, todos-show-with-done)
(todos-mode-line-function, todos-skip-archived-categories)
(todos-highlight-item, todos-wrap-lines)
(todos-line-wrapping-function): Use it.
(todos-item-insertion): New defgroup.
(todos-include-in-diary, todos-diary-nonmarking)
(todos-nondiary-marker, todos-always-add-time-string)
(todos-use-only-highlighted-region): Use it.
(todos-forward-button, todos-backward-button): New commands.
(todos-categories-mode-map): Use them, replacing forward-button
and backward-button.
(todos-merge-category): Fix and improve implementation; handle
archived items.
(todos-insert-item, todos-set-date-from-calendar): Handle setting
date by calling todos-insert-item-from-calendar.
(todos-delete-item): Fix overlay handling.
(todos-move-item): Highlight item to be moved.
(todos-item-undo): Handle marked items.
(todos-insert-item-from-calendar): Rewrite using
todos-date-from-calendar.