* calendar/todos.el: Bug fixes and improvements to item editing

* calendar/todos.el: Bug fixes and improvements to item editing
and insertion.
(todos-check-format): Compare current value of todos-categories
with actual categories sexp.
(todos-repair-categories-sexp): Add warning to doc string about
category order getting restored to list element order.
(todos-mode-external-set): When todos-categories is nil, as in
Todos Edit mode, set it by reading actual categories sexp.
(todos-edit-mode): Make buffer writeable.
(todos-done-item-section-p): New function.
(todos-insert-item): Use it as part of preventing insertion here
in done items section.  Move check for display of done items only
to just before setting new item's priority, and if cancelled after
toggling to todo items, restore display of done items.
(todos-edit-multiline-item): Don't base on todos-edit-multiline
but just narrow and change mode.
(todos-edit-multiline): Don't make indirect buffer but just widen
and change mode; also remove overlays.
(todos-edit-quit): Restore Todos mode and category display; when
quitting multiline item editing, ensure items above edited item
are visible in window if possible.
(todos-done-item-add-edit-or-delete-comment): If user moved point
during editing, make sure it moves back to edited item before
returning.