* calendar/todos.el: Fixes to todos-move-item and some of its

* calendar/todos.el: Fixes to todos-move-item and some of its
subroutines.
(todos-move-item): When there are marked items, point need not be
on an item; remove obsolete use of todos-add-category, since this
is now already done in todos-read-category; fix typo.
(todos-diary-item-p): Exclude empty lines.
(todos-read-category): Restore point and narrowing after adding
new category, to avoid moving to beginning of file when moving
marked items to a new category.
(todos-set-item-priority): Prompt for priority only when the
category has at least one todo item; only use non-nil priority to
calculate insertion location.
(todos-read-category): Don't reset todos-categories when a new
category is added due to todos-move-item or todos-jump-to-item.