Carrying units should show whether they are carrying a patient or not

[anonym-HPI]

The current image for a carrying unit looks like it never has a patient on it.

Some prototypical implementations for a general solution to visualize the number of patients in a vehicle were done in #501 #502 and #410.

@Dassderdie wasn't very happy with using different hardcoded images for the visualization. His preferred solution would be to make the problem obsolete by using a different carrying-unit image. This new image could just show the carrying unit from the side (similar to the other vehicles) and be the same with or without a patient.

[christianschaeffer]

This could be generalized to a "elements have different states with different images".
E.g. it would be useful to see whether a vehicle is full or parked, and this state could be used for permissions (i.e. can the vehicle be moved?) during an exercise.

[anonym-HPI]

This could be generalized, yes, but could get more complicated as most people would use it, clicking on a vehicle to get information, could still be a valid way (like participant walking to the vehicle and looking - not giving too much information). As you tagged it, needs discussion and so on.