ONOS-1479 - GUI Topology Overlay Work - (WIP)
- UiExtension now uses Builder Pattern; added topology overlay factory.
- Refactored UiExtensionTest (and other classes) to use builder.
- Created UiTopoOverlayFactory, UiTopoOverlay, and TopoOverlayCache.
- Started implementation of TrafficOverlay.
- Inject TopoOverlayCache into TopologyViewMessageHandler; added TopoSelectOverlay request handler.
- Modified UiExtensionManager to create traffic overlay.
- Augmented UiWebSocket to create overlays on demand, and inject overlay cache into topo view message handler.
- added client side wiring to switch overlays.

Change-Id: I6f99596aefb3b87382517ce929d268a2447545ee
diff --git a/core/api/src/main/java/org/onosproject/ui/UiTopoOverlayFactory.java b/core/api/src/main/java/org/onosproject/ui/UiTopoOverlayFactory.java
new file mode 100644
index 0000000..bd2f2fe
--- /dev/null
+++ b/core/api/src/main/java/org/onosproject/ui/UiTopoOverlayFactory.java
@@ -0,0 +1,34 @@
+/*
+ * Copyright 2015 Open Networking Laboratory
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ */
+
+package org.onosproject.ui;
+
+import java.util.Collection;
+
+/**
+ * Abstraction of an entity capable of producing one or more topology
+ * overlay handlers specific to a given user interface connection.
+ */
+public interface UiTopoOverlayFactory {
+
+    /**
+     * Produces a collection of new overlay handlers.
+     *
+     * @return collection of new overlay handlers
+     */
+    Collection<UiTopoOverlay> newOverlays();
+}