When detaching/attaching a dialog, we were only updating
appears-focused on the parent if the child itself was focused, but in
fact, we need to do it if the child has an attached child which is
focused too.

To simplify the case of detaching a focused subtree from its parent,
we change meta_window_propagate_focus_appearance() to use
@window->display->focus_window as the window to add/remove as the
attached_focus_window, and @window only as the starting point to
propagate from. That way we can propagate focus-removal up to
@window's (soon-to-be-ex-)ancestors without having to remove it from
its descendants as well.

https://bugzilla.gnome.org/show_bug.cgi?id=647712