(general) slide numbers

[ludwigbothmann]

I would like to have slide numbers since it is more easy to speak about the slides in the live sessions if one can say "on slide a/n" instead of, e.g., "on the second slide where the optimization traces are shown".

[mlindauer]

I also discussed this with Bernd and Frank some time ago, and we agreed to add them again. Please simply change the main file.