Started fleshing out the UI topo model classes.
If reviewing this, please refer to http://tinyurl.com/onos-ui-topo-model

Change-Id: I4738392bec1a89c37dff15eff6fe04d66fcabd95
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 4f2f9cb..5e6144d 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
@@ -20,4 +20,12 @@
  * Abstract base class of all elements in the UI topology model.
  */
 public class UiElement {
+
+    /**
+     * Removes all external references, and prepares the instance for
+     * garbage collection. This default implementation does nothing.
+     */
+    protected void destroy() {
+        // does nothing
+    }
 }