Fix several todo-mode bugs found while debugging bug#27609

Fix several todo-mode bugs found while debugging bug#27609

* lisp/calendar/todo-mode.el (todo-toggle-mark-item): Calculate
current category only once.
(todo-mark-category): Update number of marked items to avoid
spurious duplication in todo-categories-with-marks alist and
corruption of the todo-categories alist.  Handle empty line
when there are no todo items and done items are shown.
(todo-set-item-priority): Make noop if called from
todo-raise-item-priority or todo-lower-item-priority when
point is on a done todo item or an empty line.
(todo-move-item): Use markers instead of integer positions to
correctly handle deleting the now moved items from the source
category (without markers an infinite loop arises when moving
marked item to a preceding category).
(todo-unarchive-items): Put point on the (first) restored done
item, instead of leaving it at the end of the done items
separator string.
(todo-revert-buffer): Ensure buffer remains read-only after
reverting.