Feature: Add cargo filter support to vehicle list. (#8308)

src/widgets/group_widget.h

@@ -17,6 +17,7 @@ enum GroupListWidgets {
 	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_FILTER_BY_CARGO,          ///< Filter vehicles by cargo type.
 	WID_GL_LIST_VEHICLE,             ///< List of the vehicles.
 	WID_GL_LIST_VEHICLE_SCROLLBAR,   ///< Scrollbar for the list.
 	WID_GL_AVAILABLE_VEHICLES,       ///< Available vehicles.