Now that we're not drawing the GdkWindows by themeselves we need to
propagate into children in the same order as the windows were painted
otherwise apps can't rely on stacking order to overlay children.

This is still not 100% the same as the old behaviour, because we're
treating all windows that are part of a window at the same time (to allow
e.g. opacity groups), and we're only looking at order for the main
windows of a widget. However, this fixes at least the ordering
of the gnome-boxes fullscreen overlay toolbar.

https://bugzilla.gnome.org/show_bug.cgi?id=699970