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/timer/timer_game_tick.h

@@ -0,0 +1,34 @@
+/*
+ * This file is part of OpenTTD.
+ * OpenTTD is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, version 2.
+ * OpenTTD is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+ * See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with OpenTTD. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+/** @file timer_game_tick.h Definition of the tick-based game-timer */
+
+#ifndef TIMER_GAME_TICK_H
+#define TIMER_GAME_TICK_H
+
+#include "gfx_type.h"
+
+#include <chrono>
+
+/** Estimation of how many ticks fit in a single second. */
+static const uint TICKS_PER_SECOND = 1000 / MILLISECONDS_PER_TICK;
+
+/**
+ * Timer that represents the game-ticks. It will pause when the game is paused.
+ *
+ * @note Callbacks are executed in the game-thread.
+ */
+class TimerGameTick {
+public:
+	using TPeriod = uint;
+	using TElapsed = uint;
+	struct TStorage {
+		uint elapsed;
+	};
+};
+
+#endif /* TIMER_GAME_TICK_H */