Transfer useful additions from kinderuni-specific version

[BLuedtke]

For the Kinderuni-event, we changed/added some things in the kinderuni branch. Some of these were hardcoded scenarios.
See the comparison between the kinderuni and the master branch: master...kinderuni

The problem: I don't remember anymore which of these changes were really a good idea. To talk about a few of them:

• The getter method to cause libbidib to print the whole trackstate (link to commit): While useful to have such a printout in principle, I'm not sure if that getter is the right location for it.
• the changes in src/highlevel/bidib_highlevel_util.c: in bidib_stop, I added a section where on shutdown, all boosters (e.g., our gbm boost boards) power states are set to off. My motivation here was - I think - to make sure that when bidib_stop is performed, there's always the same power state for the boosters afterwards. I previously observed that the booster power state is affected by short circuits, and since it is not reset on bidib_stop (as I would have expected), the power state of the boosters might not be consistent/as expected on the next startup after a short in a previous run.
• the detected_kmh_speed reset in src/state/bidib_state.c (line 773) is trivial and necessary, preventing a later read from an uninitialized variable
• logging if the power state of a booster changes (link) is a good idea IMO and should be kept, as it can help debug/detected unexpected booster behaviour.
• The kinderuni scenarios integrated similarly to the physical tests (test/physical/swtbahn-full/kinderuni_suite.c and .h): I really have to admit: I don't remember if we actually used them in the event, or whether it was for prototyping only! I think in the end, we had the Kids do everything - but I have to go back and somehow see if I can find out how we did it.