[gtkmm] Window doesn't take a RefPtr for set_transient_for()

[Eric Bourque <ericb computer org>]

What is the proper way to set the transient window for a FileSelection
when get_parent_window() returns a Glib::RefPtr<Gdk::Window> and
set_transient_for() takes a Window& ?

I have a button within a composite object which displays the
FileSelection, hence the need to use get_parent_window().

Cheers,

Eric