Add NewGRF feature: Extra station name strings

These are used when all the default names have been used up,
instead of "Town Station #NNN".

src/saveload/extended_ver_sl.h

@@ -107,6 +107,7 @@ enum SlXvFeatureIndex {
 	XSLFI_CUSTOM_TOWN_ZONE,                       ///< Custom town zones
 	XSLFI_STATION_CARGO_HISTORY,                  ///< Station waiting cargo history
 	XSLFI_TRAIN_SPEED_ADAPTATION,                 ///< Train speed adaptation
+	XSLFI_EXTRA_STATION_NAMES,                    ///< Extra station names
 
 	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