[xwl] Support stack optimizing X drag source clients

Summary:
Some X clients acting as drag sources might try to optimize finding the current
target window by checking if a window manager, that sets the root window
_NET_CLIENT_LIST_STACKING property, is present. An example for this is Chromium
and since KWin sets the property the drag proxy windows must be added to this
list. Otherwise the origin client will not detect the proxy window and not send
an XdndEnter message.

Test Plan: Manually with Chromium.

Reviewers: #kwin

Subscribers: zzag, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D15628