D15628: [xwl] Support stack optimizing X drag source clients

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REVISION 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.

REPOSITORY
  R108 KWin

BRANCH
  0optimizedLayersXwl

REVISION DETAIL
  https://phabricator.kde.org/D15628

AFFECTED FILES
  layers.cpp
  workspace.h
  xwl/drag_x.cpp

To: romangg, #kwin
Cc: kwin, mkulinski, ragreen, jackyalcine, Pitel, iodelay, bwowk, ZrenBot, lesliezhai, ali-mohamed, hardening, jensreuterberg, abetts, sebas, apol, mart