Fix a segfault in the Drawing Area demo
Fix a segfault in the Drawing Area demo

* demos/gtk-demo/example_drawingarea.cc: Take a reference of event->device
in Example_DrawingArea::on_drawingarea_scribble_motion_notify_event().
RefPtr's destructor unreferences it.

GNOME/gtkmm,GNOME/gtkmm,johne53/MB3Gtkmm,GNOME/gtkmm,johne53/MB3Gtkmm,johne53/MB3Gtkmm,GNOME/gtkmm