Add reverse at waypoint orders.

The train will reverse when its tail is within the waypoint tile.
This is useful for reversing on train en-route, without creating
dedicated reversing sidings or platforms.

src/widgets/order_widget.h

@@ -29,6 +29,7 @@ enum OrderWidgets {
 	WID_O_SERVICE,                   ///< Select service (at depot).
 	WID_O_EMPTY,                     ///< Placeholder for refit dropdown when not owner.
 	WID_O_REFIT_DROPDOWN,            ///< Open refit options.
+	WID_O_REVERSE,                   ///< Select waypoint reverse type
 	WID_O_COND_VARIABLE,             ///< Choose condition variable.
 	WID_O_COND_COMPARATOR,           ///< Choose condition type.
 	WID_O_COND_VALUE,                ///< Choose condition value.