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_cmd.cpp

@@ -239,6 +239,7 @@ Order::Order(uint32 packed)
 	this->dest    = GB(packed, 16, 16);
 	this->next    = NULL;
 	this->refit_cargo   = CT_NO_REFIT;
+	this->occupancy     = 0;
 	this->wait_time     = 0;
 	this->travel_time   = 0;
 	this->max_speed     = UINT16_MAX;