refactored a lot and added possibility to play games on DedicatedServer (#43)

Backend/schafkopf-client/src/main/java/org/schafkopf/HeartbeatSender.java

@@ -0,0 +1,26 @@
+package org.schafkopf;
+
+import java.util.Timer;
+import java.util.TimerTask;
+
+/** Creates an Instance of the Backend Server. */
+public class HeartbeatSender {
+  private static final int HEARTBEAT_INTERVAL = 15000; // 1 minute
+
+  private final DedicatedServerConnection client;
+
+  public HeartbeatSender(DedicatedServerConnection client) {
+    this.client = client;
+  }
+
+  /** Creates an Instance of the Backend Server. */
+  public void start() {
+    Timer timer = new Timer();
+    timer.scheduleAtFixedRate(new TimerTask() {
+      @Override
+      public void run() {
+        client.sendMessage("HEARTBEAT SYN"); // Send a heartbeat message
+      }
+    }, HEARTBEAT_INTERVAL, HEARTBEAT_INTERVAL);
+  }
+}