(todo-insert-item-here): Prevent insertion

(todo-insert-item-here): Prevent insertion
of a new entry inside of an existing entry.  Minor code cleanup.
(todo-add-category): Change the interactive spec.  Signal an error
if the Todo file is non-empty but contains no category.  Reject
category names that could induce bugs and confusion.  Call
todo-mode if the Todo file is new and unsaved.  Simplify handling
of local variables cookie.  Properly display the newly added
category in Todo mode.
(todo-show): Call todo-initial-setup only if there is neither a
Todo file nor a corresponding unsaved buffer.
(todo-category-alist): Delete function.
(todo-completing-read): New function.
(todo-insert-item, todo-jump-to-category): Use it.
(todo-insert-item): Make the use of the prefix argument conform to
the doc string.