signal_base, signal_impl: Speed up disconnection of slots.
signal_base, signal_impl: Speed up disconnection of slots.

* sigc++/signal_base.cc: Tell signal_impl::notify() which slot is being
disconnected. Execution time is then usually O(1) instead of O(n), where n
is the size of the slot list. Disconnect all connected slots when a signal is
deleted. Disconnect slots before they are erased from a signal's slot list.
Bug #167714.

Distrotech/libsigcxx2,Distrotech/libsigcxx2,Mixaill/cmake.libsigcpp,Mixaill/cmake.libsigcpp,Mixaill/cmake.libsigcpp,Distrotech/libsigcxx2