Deal with QXcbWindowFunctions being gone in Qt6

This uses the same magic property approach as KCompletion and
Plasma Frameworks use instead.

shell/panelview.cpp

@@ -36,6 +36,7 @@
 #include <NETWM>
 #if QT_VERSION >= QT_VERSION_CHECK(6, 0, 0)
 #include <private/qtx11extras_p.h>
+#include <qpa/qplatformwindow_p.h>
 #else
 #include <QX11Info>
 #include <QtPlatformHeaders/QXcbWindowFunctions>
@@ -842,7 +843,12 @@ void PanelView::integrateScreen()
     KWindowSystem::setOnAllDesktops(winId(), true);
     KWindowSystem::setType(winId(), NET::Dock);
 #if HAVE_X11
+#if QT_VERSION < QT_VERSION_CHECK(6, 0, 0)
     QXcbWindowFunctions::setWmWindowType(this, QXcbWindowFunctions::Dock);
+#else
+    // QXcbWindow isn't installed and thus inaccessible to us, but it does read this magic property...
+    setProperty("_q_xcb_wm_window_type", QNativeInterface::Private::QXcbWindow::Dock);
+#endif
 #endif
     if (m_shellSurface) {
         m_shellSurface->setRole(KWayland::Client::PlasmaShellSurface::Role::Panel);