Signal: let setOnSubscribers be public
Signal: let setOnSubscribers be public

It is safe and it is required to make callbacks that may do things with
the signal itself.

Change-Id: Ib73c99fef2177a50513e348106927505a24a4e38
Reviewed-on: http://gerrit.aldebaran.lan/89317
Reviewed-by: vincent.palancher <f8ff75e1c61314c499fafa8de0d9e004e87e66b0@external.softbankrobotics.com>
Reviewed-by: jlamotte <70353e7b0c7d18b7fc75605daf0ccba69fcd3222@aldebaran.com>
Tested-by: vpaleologue <cb19838a79eb484e12a6960acfbf7c16dfdb80d9@aldebaran-robotics.com>

aldebaran/libqi,aldebaran/libqi,aldebaran/libqi