Feature: Option to group vehicle lists by shared orders

This applies to all kinds of vehicle lists, as well as the "vehicle groups" window.

src/widgets/vehicle_widget.h

@@ -61,6 +61,8 @@ enum VehicleDetailsWidgets {
 /** Widgets of the #VehicleListWindow class. */
 enum VehicleListWidgets {
 	WID_VL_CAPTION,                  ///< Caption of window.
+	WID_VL_GROUP_ORDER,              ///< Group order.
+	WID_VL_GROUP_BY_PULLDOWN,        ///< Group by dropdown list.
 	WID_VL_SORT_ORDER,               ///< Sort order.
 	WID_VL_SORT_BY_PULLDOWN,         ///< Sort by dropdown list.
 	WID_VL_LIST,                     ///< List of the vehicles.