Export rootWindow to the QXcbNativeInterface

This change exports the root window to the native interface as there
is QX11Info::appRootWindow which so far goes over the QDesktopWidget
to get the window id of the root window. Which is a rather hackish way
considering that the root window is known to the QXcbConnection.

But even more it's a very fragile way and can result in crashes on
startup of applications if the application accesses the appRootWindow
too early.

Change-Id: Ibb09a7fa714cb355f579298fc6df33bf80f73f58
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: David Faure <david.faure@kdab.com>
Reviewed-by: Gatis Paeglis <gatis.paeglis@digia.com>