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.cpp

@@ -153,6 +153,7 @@ const SlxiSubChunkInfo _sl_xv_sub_chunk_infos[] = {
 	{ XSLFI_CUSTOM_TOWN_ZONE,       XSCF_IGNORABLE_UNKNOWN,   1,   1, "custom_town_zone",          nullptr, nullptr, nullptr        },
 	{ XSLFI_STATION_CARGO_HISTORY,  XSCF_NULL,                1,   1, "station_cargo_history",     nullptr, nullptr, nullptr        },
 	{ XSLFI_TRAIN_SPEED_ADAPTATION, XSCF_NULL,                1,   1, "train_speed_adaptation",    nullptr, nullptr, "TSAS"         },
+	{ XSLFI_EXTRA_STATION_NAMES,    XSCF_NULL,                1,   1, "extra_station_names",       nullptr, nullptr, nullptr        },
 	{ XSLFI_NULL, XSCF_NULL, 0, 0, nullptr, nullptr, nullptr, nullptr },// This is the end marker
 };
 

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

src/saveload/station_sl.cpp

@@ -449,6 +449,7 @@ static const SaveLoad _station_desc[] = {
 	  SLE_CONDREF(Station, airport.psa,                REF_STORAGE,               SLV_161, SL_MAX_VERSION),
 
 	      SLE_VAR(Station, indtype,                    SLE_UINT8),
+	SLE_CONDVAR_X(Station, extra_name_index,           SLE_UINT16,          SL_MIN_VERSION, SL_MAX_VERSION, SlXvFeatureTest(XSLFTO_AND, XSLFI_EXTRA_STATION_NAMES)),
 
 	      SLE_VAR(Station, time_since_load,            SLE_UINT8),
 	      SLE_VAR(Station, time_since_unload,          SLE_UINT8),