Add step console command to advance n ticks

Cherry-picked from https://github.com/citymania-org/cmclient
Commit: 5ce2d21223a96934a83b8da43434c7a81f001ef0

src/openttd.h

@@ -81,6 +81,7 @@ DECLARE_ENUM_AS_BIT_SET(PauseMode)
 
 /** The current pause mode */
 extern PauseMode _pause_mode;
+extern uint32 _pause_countdown;
 
 void AskExitGame();
 void AskExitToGameMenu();