So far if the Scene creation failed kwin_wayland went into a shutdown,
but didn't succeed because the thread to start Xwayland was already
running: it froze.

This change introduces a new signal in Compositor: sceneCreated. The
startup of Xwayland is bound to this signal. If it gets fired KWin can
startup Xwayland. If it does not get fired, KWin terminates correctly.