* calendar/todos.el: Item unarchiving fixes and improvements.

* calendar/todos.el: Item unarchiving fixes and improvements.
(todos-archive-mode-map): Add bindings for item and category
marking/unmarking; make todos-display-categories binding same as
in Todos mode; remove todos-unarchive-category binding.
(todos-unarchive-category): Remove.
(todos-unarchive-items): Fix handling of marked items.  Remove
code for unarchiving whole category, since that is covered by
unarchiving marked items.  Handle unarchiving items to a category
that had been deleted from the todo file by re-adding the
category.  Use todos-category-select to display restored items,
not todos-show.  Remove obsolete or superfluous code.