Add replacement for some QX11Info alternatives to Qt6 porting guide

The porting guide at https://doc.qt.io/qt-6/extras-changes-qt6.html#changes-to-qt-x11-extras is only talking about including the QtGui/private/qtx11extras_p.h private header to be able to use QX11Info.

However, as we (thanks to David Redondo ) found out while porting the KDE Frameworks to Qt6, there are viable alternatives to some of the functionality of QX11Info that can be used instead of relying on private API:

• QX11Info::isPlatformX11() can be replaced by checking `QGuiApplication::platformName() == "xcb"

• QX11Info::display() and QX11Info::connection() can be replaced by using QGuiApplication::nativeInterface():
  auto *nativeInterface = qGuiApp->nativeInterface<QNativeInterface::QX11Application>();
  if (nativeInterface) { auto *xdisplay = nativeInterface->display(); auto *xconnection = nativeInterface->connection(); .... .... }