[gtkmm] Get parent window of any Widget for use in Dialog constructor

[Christian Neumann <christian altfrau de>]

Hi,
I need to get a reference or pointer to the root window of some
Gtk::Widget to create a transient Gtk::Dialog.
Gtk::Widget::get_parent_window() or Gtk::Widget::get_root_window() won't
work because they return a Glib::RefPtr (which I can't dereference if I
understood the docs correctly). I can't use *this because the dialog
creating function is not member of the root window.
The situation:
The Widget which creates the dialog is member of some container, which
itself is member of some other container [...]
Is there a convenient solution to this problem (other then passing the
root window pointer to each underlying widget?)?

Regards,
Christian Neumann