* calendar/todos.el: Add and revise further doc strings and

* calendar/todos.el: Add and revise further doc strings and
comments.
(todos-filter-function, todos-custom-items)
(todos-custom-items-multifile): Remove.
(todos-filter-done-items): New defcustom.
(todos-item-end, todos-backward-item): Handle todos-filter-items-mode.
(todos-filter-items): Check return value of caller first; tag
archived items for todos-jump-to-item; delete categories sexp
before processing filter; check todos-filter-done-items for
whether to leave done items; remove custom filter; fix regexp for
file and category tags; don't invoke buffer fontification.
(powerset-recursive): Fix typo.
(todos-key-bindings): Remove unused bindings.
(todos-top-priorities-multifile, todos-diary-items-multifile)
(todos-regexp-items-multifile): Use variable todos-multiple-files.
(todos-jump-to-item): Fix regexp; handle archive tags; take
todos-filter-done-items into account.
(todos-insert-item-from-calendar): Relocate to end of file
together with key-binding as addition to calendar.el.