xwm: implement _NET_CLIENT_LIST_STACKING

This property is present on all modern X11 instances. The nonpresence of
it requires applications to fall back to XQueryTree-based logic to
determine stacking logic (e.g., to determine what surface should get
Xdnd events).

These code paths are effectively untested nowadays, so this makes it
more likely for wlroots to "break" applications. For instance, the
XQueryTree fallback path has been broken in Chromium for the last 10
years.

It's easy enough to maintain this property, so let's just do it.

Fixes #2889.

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