Some of the listeners connected during the initial roundtrip assume a
non-NULL selection, when they finally invoke functions such as
gdk_wayland_selection_ensure_offer(),
gdk_wayland_selection_ensure_primary_offer() or
gdk_wayland_selection_set_offer(). The GdkWaylandSelection should
therefore be initialized before the second series of roundtrips, as the
compositor may have already sent a selection by then.