Port away from QXcbWindowFunctions for Qt6
The challenge here is that QXcbWindow::setWmWindowType is no longer accessible to us, due to the corresponding header not being installed. Looking at the implementation however shows it's reading the WM type also from a the "_q_xcb_wm_window_type" property on the window, something KCompletion is apparently already using.
Obviously I have no way to test this yet, it is possible that we do not trigger the code reading the property in all cases yet.