[sigc] Move operators in libsigc++

[Kjell Ahlstedt <kjell ahlstedt bredband net>]

I have filed bug https://bugzilla.gnome.org/show_bug.cgi?id=756484 that discusses the move constructors and move assignment operators in libsigc++. Now is your chance to comment on it, before I fix it.

We also need more methods that take an rvalue reference parameter, if we shall really benefit from the move constructors and move assignments.
    sigc::signal#<>::connect(slot_type&& slot_);
    Glib::SignalProxy#<>::connect(SlotType&& slot, bool after = true);

Kjell Ahlstedt