todo-mode.el: Fix handling of hidden item headers (bug#27609)

todo-mode.el: Fix handling of hidden item headers (bug#27609)

* lisp/calendar/todo-mode.el (todo--item-headers-hidden): New variable.
(todo-toggle-item-header): Use it.  Make this command a noop
if the file has no items.
(todo-move-item, todo-item-done): Instead of concatenating the
items to move into one string, make a list of them to
facilitate handling hidden headers.  Adjust insertion accordingly.
(todo-archive-done-item): Handle hidden headers in archive file.
(todo-unarchive-items): Handle hidden headers in todo file.
(todo-backward-item): Use todo--item-headers-hidden and handle
moving backward work when item date-time headers are hidden.
(todo-remove-item): Delete date-time header overlay.
(todo-get-overlay, todo-insert-with-overlays): Make them work
with hidden date-time headers.
(todo-modes-set-2): Make todo--item-headers-hidden buffer local.