Codechange: use better location for the "invalid" action sentinel value

It used to be a random sentinel for end-of-(widget-)list that was used to tell
that no action has taken place yet. Since the last action is practically the
widget that was pressed, add the sentinel to that enumeration.

src/widgets/rail_widget.h

@@ -28,6 +28,8 @@ enum RailToolbarWidgets {
 	WID_RAT_BUILD_TUNNEL,   ///< Build a tunnel.
 	WID_RAT_REMOVE,         ///< Bulldozer to remove rail.
 	WID_RAT_CONVERT_RAIL,   ///< Convert other rail to this type.
+
+	INVALID_WID_RAT = -1,
 };
 
 /** Widgets of the #BuildRailStationWindow class. */