* calendar/todos.el: Improve item marking and handling of marked items.

* calendar/todos.el: Improve item marking and handling of marked items.
(todos-prefix): Add validator to ensure value differs from that of
todos-item-mark.
(todos-item-mark): New defcustom.
(todos-prefix-overlay): New function.
(todos-marked-item-p): Use it.  Adapt implementation to new
handling of marked items.
(todos-insert-with-overlays): When inserting pushes down a marked
item, move its prefix overlay.
(todos-prefix-overlays): Add overlay even when prefix is empty string,
otherwise item marking fails.  Improve handling of marked items.
(todos-mark-unmark-item): Adapt to new handling of marked items
and simplify by removing marking of all items in category.
(todos-mark-category): Adapt to new handling of marked items and
don't use todos-mark-unmark-item.
(todos-unmark-category): Adapt to new handling of marked items.
(todos-delete-item): Remove obsolete handling of marked items and
useless restoration of point.
(todos-set-item-priority): Use new handling of marked items.
(todos-move-item, todos-item-done, todos-item-undo)
(todos-archive-done-item): Remove obsolete handling of marked items.