Make sure not to make the importance lower than the todos own
importance due to the its parents.

Test case and code were both wrong.

A todo becomes activated when all its children have been completed and
the other active conditions apply as well (e.g. start date has passed).

This should not be done with TodoList.delete(), since it may
remove dependency information.

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.