gs-screenshot-carousel: First ref_sink() img widget before discarding it

As the GtkWidget is an object with a floating reference, there's needed
to ref_sink() it forst before unreffing it with g_object_unref(), otherwise
there is a runtime warning on the console:

   16:42:50:751 Gtk A floating object was finalized. This means that someone
   called g_object_unref() on an object that had only a floating
   reference; the initial floating reference is not owned by anyone
   and must be removed with g_object_ref_sink().