Getting the Gtk::Window from a DrawingArea

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How does one get a Gtk::Window corresponding to a
DrawingArea? The member function get_window doesn't
return a Gtk::Window but a Gdk::Window (actually a
Glib::RefPtr<Gdk::Window>). I want a Gtk::Window
because I want to pop up a modal dialog box and the
Gtk::Dialog constructor requires a Gtk::Window not a
Gdk::Window (for the parent window of the dialog).

(Of course there is a Diaog constructor which does not
require a 'parent' parameter, but if you use that, you
get a dialog which is not necessarily placed over the
parent window, which is confusing for a modal dialog).

Nick