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/CMakeLists.txt

@@ -24,6 +24,7 @@ add_subdirectory(sound)
 add_subdirectory(spriteloader)
 add_subdirectory(table)
 add_subdirectory(tests)
+add_subdirectory(timer)
 add_subdirectory(video)
 add_subdirectory(widgets)