This object takes care of the X11 representation of the window stack,
namely the _NET_CLIENT_LIST and _NET_CLIENT_LIST_STACKING root window
properties.
This code has been lifted from src/core/stack.c into src/x11 as it's
dependent on the X11 display availability. This also leaves MetaStack
squeaky clean of x11 specifics.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/420
