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

* calendar/todos.el: Further comment revision and code
rearrangement.
(todos-item-end): Revert last change.
(todos-key-bindings, todos-menu, todos-archive-mode-map)
(todos-filter-items-mode-map): Use renamed commands.
(todos-hide-show-date-time): Rename from
todos-toggle-display-date-time.
(todos-mark-unmark-item): Rename from todos-toggle-mark-item and
adjust caller.
(todos-backward-item): Exempt special handling only from
todos-regexp-items-buffer instead of todos-filter-items-mode.
(todos-raise-item-priority): Don't allow item reprioritizing in
Todos filter items mode except for top priority items.