Updated 'ServerSener' interface.

Updated the 'ServerSender' interface by adding the 'shutdown' method to it.

Projekte/mdga/model/src/main/java/pp/mdga/server/ServerSender.java

@@ -28,4 +28,9 @@ public interface ServerSender {
      * @param id as the connection id of the client as an Integer.
      */
     void disconnectClient(int id);
+
+    /**
+     * This method will be used to shut down the server.
+     */
+    void shutdown();
 }