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/CMakeLists.txt

@@ -0,0 +1,8 @@
+add_files(
+    timer_game_calendar.cpp
+    timer_game_calendar.h
+    timer_window.cpp
+    timer_window.h
+    timer_manager.h
+    timer.h
+)