Change: replace per-AI "start_date" with a global "competitors_interval" (#10653)

The per-AI "start_date" is a lot of custom code, and was rarely
used in the way it was meant.

While at it, also ported this part over to the new timer system.

src/saveload/afterload.cpp

@@ -58,6 +58,8 @@
 #include "../disaster_vehicle.h"
 #include "../ship.h"
 #include "../water.h"
+#include "../timer/timer.h"
+#include "../timer/timer_game_tick.h"
 
 #include "saveload_internal.h"
 
@@ -3259,6 +3261,16 @@ bool AfterLoadGame()
 		for (Station *st : Station::Iterate()) UpdateStationAcceptance(st, false);
 	}
 
+	if (IsSavegameVersionBefore(SLV_AI_START_DATE)) {
+		/* For older savegames, we don't now the actual interval; so set it to the newgame value. */
+		_settings_game.difficulty.competitors_interval = _settings_newgame.difficulty.competitors_interval;
+
+		/* We did load the "period" of the timer, but not the fired/elapsed. We can deduce that here. */
+		extern TimeoutTimer<TimerGameTick> _new_competitor_timeout;
+		_new_competitor_timeout.storage.elapsed = 0;
+		_new_competitor_timeout.fired = _new_competitor_timeout.period == 0;
+	}
+
 	AfterLoadLabelMaps();
 	AfterLoadCompanyStats();
 	AfterLoadStoryBook();