Allow input systems to be disabled

Allow input systems to be disabled

To avoid trouble when running multiple instances on one machine, disable
e.g. joystick input for second instance by
--prop:bool:/sim/input/no-joystick-input=true

Also allowed:
--prop:bool:/sim/input/no-event-input=true
--prop:bool:/sim/input/no-mouse-input=true
--prop:bool:/sim/input/no-keyboard-input=true