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/OverlayService.java b/web/gui/src/main/java/org/onosproject/ui/impl/topo/OverlayService.java
index 08743ce..a046a12 100644
--- a/web/gui/src/main/java/org/onosproject/ui/impl/topo/OverlayService.java
+++ b/web/gui/src/main/java/org/onosproject/ui/impl/topo/OverlayService.java
@@ -22,6 +22,7 @@
 /**
  * Provides the API for external agents to inject topology overlay behavior.
  */
+// TODO: move to core-api module
 public interface OverlayService {
 
     /**