core: TracedCallback connect status

There is no way to know if a TracedCallback has at least one connected callback, which could be handy sometimes.

The use-case is:

• Do some very complex stuff to prepare the data for the TracedCallback.
• Fire the TracedCallback (but there's nothing in it).
• Bummer.

It would be enough to have a:

template <typename... Ts>
bool
TracedCallback<Ts...>::IsEmpty () const
{
  return m_callbackList.empty ();
}