Signs: Max30, Max60, Max90

Cars slow down at speed limit signs
Bug fix

Fixed bugs in car passing, cars know the car in front and in back, to
properly switch car pointers between them
Bug fixes

Elegantly fixed a bug of dividing by NaN, using the fact that
comparisons with NaN result in false
Fixed another bug of crossroad priorities, using the variable passCar
later
Better GUI

Fixed loading new saves when the simulation is running, also added
support for pausing the sim