Order occupancy: Add column to orders GUI to show occupancy running average.

This is an exponentially weighted moving average of occupancies updated
when any vehicle in the shared order set leaves the station of that order.
The weighting factor is an adv setting.

src/settings_type.h

@@ -439,6 +439,7 @@ struct OrderSettings {
 	bool   selectgoods;                      ///< only send the goods to station if a train has been there
 	bool   no_servicing_if_no_breakdowns;    ///< don't send vehicles to depot when breakdowns are disabled
 	bool   serviceathelipad;                 ///< service helicopters at helipads automatically (no need to send to depot)
+	uint8  occupancy_smoothness;             ///< percentage smoothness of occupancy measurement changes
 };
 
 /** Settings related to vehicles. */