Support free structure reference in timers

[denis-stepanov]

Sometimes it might be convenient to store additional information with the timer, which could be needed when timer action is executed. One example is when a timer happens to be part of a class, a pointer to that class instance could be stored; then, a handler could call methods of the class the timer belongs to. Currently this scenario requires storing additional information in a global area. A pointer should be of type void * and by default initialized to nullptr.