Starting at 61b2bd5e, boost::bind was progressively replaced with
std::bind. They are not interchangeable though. boost::bind implements
the tracking of boost::signals{,2}::trackable objects. Now that
std::bind has completely replaced boost::bind, tracking never occurred.
This commit replaces boost::signals2::trackable with the new preferred
boost::signals2 methods: scoped_connections or slot::track_foreign. The
support::Trackable class introduced is less safe but easier for transitioning
old code.
Fixes the crash at #8261.
