Only start Xwayland server if Compositor created a Scene

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.

composite.cpp

@@ -278,6 +278,7 @@ void Compositor::slotCompositingOptionsInitialized()
         }
         return;
     }
+    emit sceneCreated();
 
     if (Workspace::self()) {
         startupWithWorkspace();

composite.h

@@ -186,6 +186,7 @@ public Q_SLOTS:
 Q_SIGNALS:
     void compositingToggled(bool active);
     void aboutToDestroy();
+    void sceneCreated();
 
 protected:
     void timerEvent(QTimerEvent *te);

main_wayland.cpp

@@ -18,6 +18,7 @@ You should have received a copy of the GNU General Public License
 along with this program.  If not, see <http://www.gnu.org/licenses/>.
 *********************************************************************/
 #include "main_wayland.h"
+#include "composite.h"
 #include "workspace.h"
 #include <config-kwin.h>
 // kwin
@@ -152,8 +153,7 @@ void ApplicationWayland::continueStartupWithScreens()
         return;
     }
     createCompositor();
-
-    startXwaylandServer();
+    connect(Compositor::self(), &Compositor::sceneCreated, this, &ApplicationWayland::startXwaylandServer);
 }
 
 void ApplicationWayland::continueStartupWithX()
@@ -280,6 +280,7 @@ void ApplicationWayland::createX11Connection()
 
 void ApplicationWayland::startXwaylandServer()
 {
+    disconnect(Compositor::self(), &Compositor::sceneCreated, this, &ApplicationWayland::startXwaylandServer);
     int pipeFds[2];
     if (pipe(pipeFds) != 0) {
         std::cerr << "FATAL ERROR failed to create pipe to start Xwayland " << std::endl;