Add button to toggle train purchase window dual pane mode

src/vehicle_gui.h

@@ -57,6 +57,7 @@ void DrawShipImage(const Vehicle *v, const Rect &r, VehicleID selection, EngineI
 void DrawAircraftImage(const Vehicle *v, const Rect &r, VehicleID selection, EngineImageType image_type);
 
 void ShowBuildVehicleWindow(TileIndex tile, VehicleType type);
+void ShowTemplateTrainBuildVehicleWindow(Train **virtual_train);
 
 uint ShowRefitOptionsList(int left, int right, int y, EngineID engine);
 StringID GetCargoSubtypeText(const Vehicle *v);