Feature: [Linkgraph] Prioritize faster routes for passengers, mail and express cargo

Passengers usually prefer fast paths to short paths.
Average travel times of links are updated in real-time for use in Dijkstra's algorithm,
and newer travel times weigh more, just like capacities.

(cherry picked from commit 977604ef08)

src/saveload/extended_ver_sl.cpp

@@ -178,6 +178,7 @@ const SlxiSubChunkInfo _sl_xv_sub_chunk_infos[] = {
 	{ XSLFI_TOWN_SETTING_OVERRIDE,  XSCF_NULL,                1,   1, "town_setting_override",     nullptr, nullptr, nullptr        },
 	{ XSLFI_SCRIPT_INT64,           XSCF_NULL,                1,   1, "script_int64",              nullptr, nullptr, nullptr        },
 	{ XSLFI_U64_TICK_COUNTER,       XSCF_NULL,                1,   1, "u64_tick_counter",          nullptr, nullptr, nullptr        },
+	{ XSLFI_LINKGRAPH_TRAVEL_TIME,  XSCF_NULL,                1,   1, "linkgraph_travel_time",     nullptr, nullptr, nullptr        },
 	{ XSLFI_NULL, XSCF_NULL, 0, 0, nullptr, nullptr, nullptr, nullptr },// This is the end marker
 };