Config:systemd: Invoke systemd notify when the compositor is ready

Now the compositor calls systemd notify READY=1 when surfaces can be created. This allows the use of a systemd notify type start script.

BCS 746-000824

multiscreen-compositor/qml/main.qml

@@ -110,6 +110,15 @@ WaylandCompositor {
         onIviSurfaceCreated: handleIviSurfaceCreated(iviSurface)
     }
 
+    // =============================================
+    // We need to notify systemd when the compositor
+    // is ready to create surfaces
+    // =============================================
+    onCreatedChanged: {
+        if (created === true) {
+            config.notifyCompositorReady();
+        }
+    }
 
     onSurfaceRequested: {
        console.log("Surface requested", client, id, version);