xcb: set _NET_STARTUP_ID at client leader window
requested to merge asturmlechner/qtbase:asturmlechner/qtgui-QTBUG-96276-_NET_STARTUP_ID into kde/5.15
This should help to overcome WM's focus prevention mechanism
Fixes: QTBUG-96276 Pick-to: 6.4 6.3 6.2 5.15 Change-Id: Ic5fb46f7ce54f0df29850725bafa364b74e30d25 Reviewed-by: Qt CI Bot qt_ci_bot@qt-project.org Reviewed-by: Liang Qi liang.qi@qt.io (cherry picked from commit ba8c4b4a)