* calendar/todos.el: Item insertion and done/undo fixes.

* calendar/todos.el: Item insertion and done/undo fixes.
(todos-insert-item): Maybe set the insertion file to
todos-global-current-todos-file.  When inserting into a file that
was not being visited on invoking this command, ensure chosen
category is properly selected.  Make item inserted "here" the
first todo item if the command was invoked with point in done
items section or outside of the current category.
(todos-item-done): When done items are shown, put cursor on first
just done item.
(todos-item-undo): Put cursor on undone item.