Add new signal type: no-entry signal

This is only passable in one direction, but does not have a signal
or show an aspect in the opposite direction

Add a setting for whether this is shown in the signal UI.
Off by default.

src/saveload/extended_ver_sl.cpp

@@ -155,6 +155,7 @@ const SlxiSubChunkInfo _sl_xv_sub_chunk_infos[] = {
 	{ 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_DEPOT_ORDER_EXTRA_FLAGS,XSCF_IGNORABLE_UNKNOWN,   1,   1, "depot_order_extra_flags",   nullptr, nullptr, nullptr        },
+	{ XSLFI_EXTRA_SIGNAL_TYPES,     XSCF_NULL,                1,   1, "extra_signal_types",        nullptr, nullptr, nullptr        },
 	{ XSLFI_NULL, XSCF_NULL, 0, 0, nullptr, nullptr, nullptr, nullptr },// This is the end marker
 };
 

src/saveload/extended_ver_sl.h

@@ -109,6 +109,7 @@ enum SlXvFeatureIndex {
 	XSLFI_TRAIN_SPEED_ADAPTATION,                 ///< Train speed adaptation
 	XSLFI_EXTRA_STATION_NAMES,                    ///< Extra station names
 	XSLFI_DEPOT_ORDER_EXTRA_FLAGS,                ///< Depot order extra flags
+	XSLFI_EXTRA_SIGNAL_TYPES,                     ///< Extra signal types
 
 	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