Also guard the set_priority method in TodoList a bit more, to
prevent the dirty bit to be set when an equal priority is passed
on.

Otherwise we get useless trailing newlines in our output.

Also modify the keys for the dependency graph, use the hashed version of
the todo instead of its position.
The position may shift when deleting tasks, which affects the consistency
of the dependency graph.

Now, the todolist is the owner of the numbers.

A step towards elimating knowledge of the todo list in the items itself.