Find out if a widget or window is actually visible?

[Toralf Lund <toralf procaptura com>]

Does Gtkmm/Gdkmm offer a nice and simple way to find out if a widget or 
window is actually visible to the user, i.e. is mapped *and not obscured 
by another window*? I mean, .e.g Gdk::Window::is_viewable () and 
Gdk::Window::is_visible ()/Gtk::Widget::is_visible() will answer only 
first half of that question, I believe, i.e. they will tell me whether 
the window/widget is mapped, but not check if it is covered by something 
else.

- Toralf