Scheduled Dispatch feature

Code is tested and run well on small-ish train network. Not tested extensively.

src/saveload/extended_ver_sl.h

@@ -50,6 +50,7 @@ enum SlXvFeatureIndex {
 	XSLFI_STATION_CATCHMENT_INC,                  ///< Station catchment radius increase
 	XSLFI_CUSTOM_BRIDGE_HEADS,                    ///< Custom bridge heads
 	XSLFI_CHUNNEL,                                ///< Tunnels under water (channel tunnel)
+	XSLFI_SCHEDULED_DISPATCH,                     ///< Scheduled vehicle dispatching
 
 	XSLFI_RIFF_HEADER_60_BIT,                     ///< Size field in RIFF chunk header is 60 bit
 	XSLFI_HEIGHT_8_BIT,                           ///< Map tile height is 8 bit instead of 4 bit, but savegame version may be before this became true in trunk