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/group_widget.h

@@ -13,6 +13,8 @@
 /** Widgets of the #VehicleGroupWindow class. */
 enum GroupListWidgets {
 	WID_GL_CAPTION,                  ///< Caption of the window.
+	WID_GL_GROUP_BY_ORDER,           ///< Group order.
+	WID_GL_GROUP_BY_DROPDOWN,        ///< Group by dropdown list.
 	WID_GL_SORT_BY_ORDER,            ///< Sort order.
 	WID_GL_SORT_BY_DROPDOWN,         ///< Sort by dropdown list.
 	WID_GL_LIST_VEHICLE,             ///< List of the vehicles.