Make the sequence order of callbacks to observers of the same event predictable

[andreas-0815-qwertz]

As observers are stored in a plain dict the order of callbacks is only predictable since Python 3.7, i.e. it will call back in the order of calls to add_observer. It would be nice to have an more explicit way to specify the order of callbacks in general, and specifically for older Python versions. The latter case is unimportant for me personally though.