Feature #8129: Add operator+ and operator- to qi::os::timeval

Change-Id: I6d418a27c37e9dee54e5956387521e066ae6a06c
Reviewed-on: http://gerrit.aldebaran.lan:8080/10465
Reviewed-by: ncornu <ncornu@aldebaran-robotics.com>
Reviewed-by: cgestes <gestes@aldebaran-robotics.com>
Reviewed-by: mnottale <mnottale@aldebaran-robotics.com>
Tested-by: hcuche <hcuche@aldebaran-robotics.com>