Fix the Change Display demo
Fix the Change Display demo

Use GestureMultiPress instead of the deprecated Widget::signal_event().
Make the demo more like the corresponding demo in gtk+, e.g. by using
Gdk::Seat::grab() and Gdk::Device::get_window_at_position().

GNOME/gtkmm,GNOME/gtkmm,GNOME/gtkmm,GNOME/gtkmm