Survive Xwayland crashes

At the moment, Xwayland will bring down the entire session with itself if it crashes. We don't really want that. If Xwayland crashes, only X11 clients must be killed.

In order to make the compositor resilient to Xwayland crashes, relevant parts in kwin were refactored to allow starting and stopping the X11 machinery at any particular moment.

Also, the new start() and stop() methods can become very handy if we decide that it would be pretty cool to restart Xwayland when it crashes. Implementing that should be a matter of adding a new option and a few lines of code in Xwayland::handleXwaylandFinished() (a stop() followed by a start()).

Closes #8 (closed)