Folding vehicles

Fixes #6966

This interferes with #7020, therefor it's marked as WIP, but it's technically ready. I'm going to rebase this or the other PR when needed. Also some strings must be changed (made generic/get ride of the bicycle references) but this interferes with the other PR, too.

During merging one might to get ride of the wheelchair, as it is non-functional. It has no pedals (which would be stupid anyway), and "hand pedals" don't exist. So its basically a shopping cart with a seat that can be pushed / pulled. It's only there to test the folding functions because it consists of foldable parts only.

Also related to #3251