[boost] [Signals] Added disconnect() based on function object equivalence

Since we're discussing the issue, I've added an experimental disconnect() member function to signals that allows one to disconnect equivalent slots, e.g., void foo(); void bar(); signal<void()> sig; sig.connect(&foo); sig.connect(&bar); sig.disconnect(&foo); // disconnects foo, but not bar We could also reconsider the += and -= syntax, since support for -= now makes some sense. Doug