display: Do not include unmanaging windows in list_windows()

There's a small window before a window that is being unmanaged is
unregistered with the display. The MetaScreen::window-left-monitor
and MetaWorkspace::window-removed emissions fall right into that
window, so code that runs in that time may well be out of our
control; we can make sure that the method it can use to get an
updated list of windows no longer contains the destroyed window
though, which is a much better option than expecting everyone to
filter the list themselves.