Codechange: introduce a framework for all our timers

IntervalTimer and TimeoutTimer use RAII, and can be used to replace
all the time-based timeouts, lag-detection, "execute every N" we
have.
As it uses RAII, you can safely use it as static variable, class
member, temporary variable, etc. As soon as it goes out-of-scope,
it will be safely removed.
This allows for much easier to read code when it comes to intervals.

src/timer/timer_window.h

@@ -0,0 +1,35 @@
+/*
+ * 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_window.h Definition of the Window system */
+
+#ifndef TIMER_WINDOW_H
+#define TIMER_WINDOW_H
+
+#include <chrono>
+
+/**
+ * Timer that represents the real time, usable for the Window system.
+ *
+ * This can be used to create intervals based on milliseconds, seconds, etc.
+ * Mostly used for animation, scrolling, etc.
+ *
+ * Please be mindful that the order in which timers are called is not guaranteed.
+ *
+ * @note The lowest possible interval is 1ms.
+ * @note These timers can only be used in the Window system.
+ */
+class TimerWindow {
+public:
+	using TPeriod = std::chrono::milliseconds;
+	using TElapsed = std::chrono::milliseconds;
+	struct TStorage {
+		std::chrono::milliseconds elapsed;
+	};
+};
+
+#endif /* TIMER_WINDOW_H */