[sigc] std::function<>-style declarations

[Murray Cumming <murrayc murrayc com>]

I'd like to move libsigc++ gradually to a
  sigc::signal<void(int, Thing)>
syntax, instead of
  sigc::signal<void, int, Thing>
and for sigc::slot, because std::function has now established that. We
would deprecate the old syntax in libsigc++-2.0, but only remove it in
libsigc++-3.0.

Objections or thoughts about how to do this with less code?
https://bugzilla.gnome.org/show_bug.cgi?id=763393

-- 
Murray Cumming
murrayc murrayc com
www.murrayc.com