* calendar/todos.el: Significant code rearrangement; further

* calendar/todos.el: Significant code rearrangement; further
comment revision.
(todos-filtered-items-buffer): Rename from todos-filter-buffer and
adjust users.
(todos-filtered-buffer-name): Rename from
todos-special-buffer-name and adjust users.
(todos-filtered-items-mode-map): Rename from
todos-filter-items-mode-map and adjust users.
(todos-mode-external-set): Use todos-categories instead of
todos-set-categories (and add comment to check if this DTRT).
(todos-filtered-items-mode): Rename from todos-filter-items-mode
and adjust users.
(todos-add-category): Don't call todos-validate-name, since
todos-read-category does.
(todos-edit-quit): Use todos-repair-categories-sexp.
(todos-done-item-add-edit-or-delete-comment): Rename from
todos-done-item-add-or-edit-comment and adjust users; add optional
argument to prompt to delete comment.
(todos-item-undo): Delete done item comment on undoing if user
confirms.