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/order_base.h

@@ -43,6 +43,8 @@ private:
 
 	CargoID refit_cargo;  ///< Refit CargoID
 
+	uint8 occupancy;     ///< Estimate of vehicle occupancy on departure, for the current order, 0 indicates invalid, 1 - 101 indicate 0 - 100%
+
 	uint16 wait_time;    ///< How long in ticks to wait at the destination.
 	uint16 travel_time;  ///< How long in ticks the journey to this destination should take.
 	uint16 max_speed;    ///< How fast the vehicle may go on the way to the destination.
@@ -217,6 +219,18 @@ public:
 	 */
 	inline void SetMaxSpeed(uint16 speed) { this->max_speed = speed; }
 
+	/**
+	 * Get the occupancy value
+	 * @return occupancy
+	 */
+	inline uint8 GetOccupancy() const { return this->occupancy; }
+
+	/**
+	 * Set the occupancy value
+	 * @param occupancy The occupancy to set
+	 */
+	inline void SetOccupancy(uint8 occupancy) { this->occupancy = occupancy; }
+
 	bool ShouldStopAtStation(const Vehicle *v, StationID station) const;
 	bool CanLoadOrUnload() const;
 	bool CanLeaveWithCargo(bool has_cargo) const;