Factor timestep calculation out to Game function

This is so we can grab the game's timestep anywhere else in the codebase
without copy-pasting it.

desktop_version/src/Game.h

@@ -292,6 +292,7 @@ public:
     bool  colourblindmode;
     bool noflashingmode;
     int slowdown;
+    int get_timestep(void);
 
     bool nodeathmode;
     int gameoverdelay;