* calendar/todos.el: Further code rearrangement and comment

* calendar/todos.el: Further code rearrangement and comment
revision.
(todos-item-start): Handle empty line between todo and done items
when done items are displayed.
(todos-key-bindings): Comment out bindings meant only for
todos-archive-mode.
(todos-archive-mode-map): Fix typo.
(todos-archive-mode): Derive from special-mode instead of
todos-mode to prevent its key bindings from being available here.
(todos-archive-done-item): Remove obsolete code; fix item count
updating.
(todos-unarchive-items): Simplify; fix unarchiving of all items in
category; fix item count updating; fix typo.