libsigc++: New API in libsigc++ 2.4

Member sigc::signal_base::block (bool should_block=true) noexcept
Member sigc::signal_base::blocked () const noexcept
Member sigc::signal_base::unblock () noexcept
Member sigc::track_obj (const T_functor& func, const T_obj&...obj)
Class sigc::track_obj_functor< T_functor, T_obj >
Module track_obj