Use p3 for order indices which don't fit into p1

Fix insert/modify order overflowing p1
Don't use text field for change timetable

src/script/api/script_order.hpp

@@ -600,6 +600,9 @@ public:
 	 * @see ScriptEngine::GetMaximumOrderDistance and ScriptVehicle::GetMaximumOrderDistance
 	 */
 	static uint GetOrderDistance(ScriptVehicle::VehicleType vehicle_type, TileIndex origin_tile, TileIndex dest_tile);
+
+protected:
+	static bool ScriptOrderModifyOrder(VehicleID vehicle_id, VehicleOrderID order_position, uint32 p2, Script_SuspendCallbackProc *callback = nullptr);
 };
 DECLARE_ENUM_AS_BIT_SET(ScriptOrder::ScriptOrderFlags)