GUI -- Further work on refactoring Topology View server side code. Still WIP...
- added topology client heartbeat.
- modified AbstractListenerRegistry to allow for extension.

Change-Id: Ib8ea6ad4ba34f5732d062f1c9ef545f105eb167b
diff --git a/web/gui/src/main/java/org/onosproject/ui/impl/topo/SummaryData.java b/web/gui/src/main/java/org/onosproject/ui/impl/topo/SummaryData.java
index 6dd93ee..c53d82d 100644
--- a/web/gui/src/main/java/org/onosproject/ui/impl/topo/SummaryData.java
+++ b/web/gui/src/main/java/org/onosproject/ui/impl/topo/SummaryData.java
@@ -20,6 +20,7 @@
 /**
  * Provides basic summary data for the topology.
  */
+// TODO: review -- move to core-api module?
 public interface SummaryData {
 
     /**