ONOS-4326: Focusing on add/remove cluster member. (WIP).

If reviewing this, please refer to http://tinyurl.com/onos-ui-topo-model

Change-Id: Ic6568074ac768ec828f9103e92caab5e9a06ade6
diff --git a/core/api/src/main/java/org/onosproject/ui/model/topo/UiElement.java b/core/api/src/main/java/org/onosproject/ui/model/topo/UiElement.java
index 5e6144d..f0c2684 100644
--- a/core/api/src/main/java/org/onosproject/ui/model/topo/UiElement.java
+++ b/core/api/src/main/java/org/onosproject/ui/model/topo/UiElement.java
@@ -19,7 +19,7 @@
 /**
  * Abstract base class of all elements in the UI topology model.
  */
-public class UiElement {
+public abstract class UiElement {
 
     /**
      * Removes all external references, and prepares the instance for
@@ -28,4 +28,11 @@
     protected void destroy() {
         // does nothing
     }
+
+    /**
+     * Returns a string representation of the element identifier.
+     *
+     * @return the element unique identifier
+     */
+    public abstract String idAsString();
 }