* todos.el: More code cleanup. Update file copyright and author

* todos.el: More code cleanup.  Update file copyright and author
lines, add commentary.  Reformat so most lines do not exceed 80
columns.  Fix doc strings problems found by checkdoc.
(todos-edit-file): Delete unused argument.
(todos-delete-item): Delete superfluous concat.
(todos-next-button, todos-previous-button): Delete optional
arguments, adjusting interactive spec accordingly, and add them as
non-nil arguments to the invocations of forward-button and
backward-button.