2.99.7: (unstable)

* connection, slot_base: Use an internal::weak_raw_ptr<> to simplify
  the code.
  (Murray Cumming)
* signal_base: Do not derive from trackable.
  (Murray Cumming
*  slot_rep: Pass a slot_rep*, not notifiable*, to destroy().
  (Murray Cumming)
* signal_impl: Do not derive from notifiable.
  (Murray Cumming)