GUI2 Handle node additions and removals in Topology view

Change-Id: Ic16fc1325fe338e2136f1cc70febc621342be4f2
diff --git a/core/api/src/main/java/org/onosproject/ui/model/topo/UiHost.java b/core/api/src/main/java/org/onosproject/ui/model/topo/UiHost.java
index 36375eb..5ca86e3 100644
--- a/core/api/src/main/java/org/onosproject/ui/model/topo/UiHost.java
+++ b/core/api/src/main/java/org/onosproject/ui/model/topo/UiHost.java
@@ -73,6 +73,16 @@
     }
 
     /**
+     * Returns the identifier of the region to which this device belongs.
+     * This will be null if the device does not belong to any region.
+     *
+     * @return region ID
+     */
+    public RegionId regionId() {
+        return regionId;
+    }
+
+    /**
      * Sets the ID of the region to which this device belongs.
      *
      * @param regionId region identifier