Gtk glass backend should run with Gtk+ 3.8 (minimum)

on gladd_dnd.cpp:

#if GTK_CHECK_VERSION(3, 10, 0)
    context = gtk_drag_begin_with_coordinates(drag_widget, tlist, actions, 1, NULL, x, y);
#else
    context = gtk_drag_begin(drag_widget, tlist, actions, 1, NULL);
#endif


This will break if run on previous Gtk than 3.10.0.