fix: make OrgChartState serializable

Close #76

Author: javier-godoy

src/main/java/com/flowingcode/vaadin/addons/orgchart/client/OrgChartState.java

@@ -22,10 +22,11 @@
 
 import com.flowingcode.vaadin.addons.orgchart.client.constants.ChartConstants;
 import com.flowingcode.vaadin.addons.orgchart.client.enums.ChartDirectionEnum;
+import java.io.Serializable;
 
 /** @author pbartolo */
 @SuppressWarnings("serial")
-public class OrgChartState {
+public class OrgChartState implements Serializable {
 
   public String value;