When meta_display_unmanage_window_for_screen() is called, it gets a list
of windows and iterates over them and unmanages them, but unmanaging a
window with attached modal dialogs also unmanages those attached modal
dialogs (in the normal case, temporarily), so we need to guard against
such cases by ref'ing the windows in the list and checking if they have
already been unmanaged.
https://bugzilla.gnome.org/show_bug.cgi?id=668299https://bugzilla.redhat.com/show_bug.cgi?id=760918
c0a440b852