Only show ship is lost messages if lost for a significant time

src/saveload/extended_ver_sl.cpp

@@ -127,6 +127,7 @@ const SlxiSubChunkInfo _sl_xv_sub_chunk_infos[] = {
 	{ XSLFI_DOCKING_CACHE_VER,      XSCF_IGNORABLE_ALL,       1,   1, "docking_cache_ver",         nullptr, nullptr, nullptr        },
 	{ XSLFI_EXTRA_CHEATS,           XSCF_NULL,                1,   1, "extra_cheats",              nullptr, nullptr, "CHTX"         },
 	{ XSLFI_TOWN_MULTI_BUILDING,    XSCF_NULL,                1,   1, "town_multi_building",       nullptr, nullptr, nullptr        },
+	{ XSLFI_SHIP_LOST_COUNTER,      XSCF_NULL,                1,   1, "ship_lost_counter",         nullptr, nullptr, nullptr        },
 	{ XSLFI_NULL, XSCF_NULL, 0, 0, nullptr, nullptr, nullptr, nullptr },// This is the end marker
 };
 

src/saveload/extended_ver_sl.h

@@ -84,6 +84,7 @@ enum SlXvFeatureIndex {
 	XSLFI_DOCKING_CACHE_VER,                      ///< Multiple docks - docking tile cache version
 	XSLFI_EXTRA_CHEATS,                           ///< Extra cheats
 	XSLFI_TOWN_MULTI_BUILDING,                    ///< Allow multiple stadium/church buildings in a single town
+	XSLFI_SHIP_LOST_COUNTER,                      ///< Ship lost counter
 
 	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/vehicle_sl.cpp

@@ -840,6 +840,7 @@ const SaveLoad *GetVehicleDescription(VehicleType vt)
 		      SLE_VAR(Ship, state,                     SLE_UINT8),
 		SLE_CONDDEQUE(Ship, path,                      SLE_UINT8,                  SLV_SHIP_PATH_CACHE, SL_MAX_VERSION),
 		  SLE_CONDVAR(Ship, rotation,                  SLE_UINT8,                  SLV_SHIP_ROTATION, SL_MAX_VERSION),
+		SLE_CONDVAR_X(Ship, lost_count,                SLE_UINT8,                     SL_MIN_VERSION, SL_MAX_VERSION, SlXvFeatureTest(XSLFTO_AND, XSLFI_SHIP_LOST_COUNTER)),
 
 		SLE_CONDNULL(16, SLV_2, SLV_144), // old reserved space