Add new variants of push_change_event() with w_value argument

It would be nice to have a push_change_event() method with the w_value argument. Otherwise one needs to create a critical section including both calls: set_write_value() and push_change_event() by applying the Tango serilization monitor (AutoTangoMonitor() context manager in PyTango).

See discussion in this Tango forum thread for more details.