wm-spec: Add _NET_WM_WINDOW_OPACITY
This was proposed back in 2003 by Keith Packard at
https://lists.freedesktop.org/archives/xdg/2003-December/001413.html
but never made it into the spec. Both Qt and GTK as well as other toolkits and wm's use it though so I figured it might make sense to at least have an MR.