Add missing @since to VDragAndDropManager

Change-Id: Icf4bc147b2ca77676ce29c7b2b8a516cbf7b2d23

client/src/com/vaadin/client/ui/dd/VDragAndDropManager.java

@@ -63,6 +63,8 @@ public class VDragAndDropManager {
      * Implementation if this interface is provided as a parameter to
      * DDEventHandleStrategy methods. The mediator instance allows to manage
      * DnD.
+     * 
+     * @since 7.4.4
      */
     public interface DDManagerMediator {
         /**