* calendar/todos.el: Further comment revision.

* calendar/todos.el: Further comment revision.
(todos-reset-global-current-todos-file):
Try to make this not slow down kill-buffer.
(todos-update-categories-sexp): Handle the case where there is no
categories sexp yet, i.e. after inserting the first item in the
file, so todos-display-categories works.
(todos-read-file-name): Improve implementation.
(todos-validate-name): Use variable todos-files.
(todos-category-number): New variable.
(todos-insert-category-line, todos-update-categories-display)
(todos-raise-category-priority): Use it.
(todos-add-file): Remove unused remnant code.