* calendar/todos.el: Several small fixes and improvements.

* calendar/todos.el: Several small fixes and improvements.
(todos-done-separator-string): Change default value.
(todos-update-categories-sexp): Remove unnecessary and misquided
setq of todos-default-todos-file.  Use delete-region instead of
kill-region to avoid polluting kill-ring.
(todos-unload-hook): Remove (was not used).
(todos-edit-multiline): Don't generate new edit buffer name on
each invocation---that can cause confusion due to indirect buffer.
(todos-edit-quit): Ensure lines in edit buffer that follow hard
newlines are indented in the file to conform to diary format.
(todos-set-item-priority): If done items in category are visible,
keep them visible.