+ - Every signal now come with a static version that is invoked for every object of that class,
+ and an instance version that is invoked for this specific object only. For example,
+ s4u::Actor::on_suspend_cb() adds a callback that is invoked for the suspend of any actor while
+ s4u::Actor::on_this_suspend_cb() adds a callback for this specific actor only.
+ - Activity::on_suspended_cb() is renamed to Activity::on_suspend_cb(), and fired right before the suspend.
+ - Activity::on_resumed_cb() is renamed to Activity::on_resume_cb(), and fired right before the resume.
+ - Resource::on_state_change_cb() is renamed to Resource::on_onoff_cb() to distinguish from the
+ Activity::on_state_change_cb() that is related to the activity state machine, not on/off.
+ - Activity signals (veto, suspend, resume, completion) are now specialized by activity class.
+ That is, callbacks registered in Exec::on_suspend_cb will not be fired for Comms nor Ios.