Codechange: Use begin/end of nwidget parts of begin/length.

This simplifies processing nwidget parts as, unlike the remaining length, the pointer to the end of the list never changes. This is the same principle as we use(d) for tracking end instead of length for C-style strings.

And this removes 160~ instances of the lengthof() macro.

src/order_gui.cpp

@@ -1640,7 +1640,7 @@ static WindowDesc _orders_train_desc(
 	WDP_AUTO, "view_vehicle_orders_train", 384, 100,
 	WC_VEHICLE_ORDERS, WC_VEHICLE_VIEW,
 	WDF_CONSTRUCTION,
-	_nested_orders_train_widgets, lengthof(_nested_orders_train_widgets),
+	std::begin(_nested_orders_train_widgets), std::end(_nested_orders_train_widgets),
 	&OrdersWindow::hotkeys
 );
 
@@ -1713,7 +1713,7 @@ static WindowDesc _orders_desc(
 	WDP_AUTO, "view_vehicle_orders", 384, 100,
 	WC_VEHICLE_ORDERS, WC_VEHICLE_VIEW,
 	WDF_CONSTRUCTION,
-	_nested_orders_widgets, lengthof(_nested_orders_widgets),
+	std::begin(_nested_orders_widgets), std::end(_nested_orders_widgets),
 	&OrdersWindow::hotkeys
 );
 
@@ -1740,7 +1740,7 @@ static WindowDesc _other_orders_desc(
 	WDP_AUTO, "view_vehicle_orders_competitor", 384, 86,
 	WC_VEHICLE_ORDERS, WC_VEHICLE_VIEW,
 	WDF_CONSTRUCTION,
-	_nested_other_orders_widgets, lengthof(_nested_other_orders_widgets),
+	std::begin(_nested_other_orders_widgets), std::end(_nested_other_orders_widgets),
 	&OrdersWindow::hotkeys
 );