mouse.signal_button_release.connect(sigc::bind_return(sigc::mem_fun(this, &Viewer::button_release), false));
mouse.signal_axis_motion.connect(sigc::bind_return(sigc::mem_fun(this, &Viewer::axis_motion), false));
mouse.signal_button_release.connect(sigc::bind_return(sigc::mem_fun(this, &Viewer::button_release), false));
mouse.signal_axis_motion.connect(sigc::bind_return(sigc::mem_fun(this, &Viewer::axis_motion), false));