Creates a socketpair in WaylandServer and creates a ClientConnection for
Xwayland. The created file descriptor is passed to Xwayland through the
WAYLAND_SOCKET env variable.
The WaylandServer is at the moment only used to support starting an
Xwayland. It does not support Wayland clients yet, so don't get
excited.
For Xwayland it's using the trick of creating the Display before the
QApplication is created with manual event dispatching.