input_server must be notified about shutdown

[marcusoverhagen]

input server must be notified that the system is beeing shut down, as it has to notify BInputServerDevice::SystemShuttingDown()

BeBook:

Tells the object that the Input Server is in the process of shutting down. Unless something interrupts the shutdown, this notification will be followed by a Stop() and delete, thus you don't have to do much from this function (other than note that the end is near).