Adding some base-classes to eliminate event and listener boiler-plate code throughout a number of subsystems.

Refactored all core components to take advantage of this; apps remain to be done.

Change-Id: Ib0935ba07ff81b0fa032534004ec9ac6187cbf22
diff --git a/core/api/src/main/java/org/onosproject/event/ListenerService.java b/core/api/src/main/java/org/onosproject/event/ListenerService.java
new file mode 100644
index 0000000..a4a3631
--- /dev/null
+++ b/core/api/src/main/java/org/onosproject/event/ListenerService.java
@@ -0,0 +1,38 @@
+/*
+ * 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.event;
+
+/**
+ * Abstraction of a service capable of asynchronously notifying listeners.
+ */
+public interface ListenerService<E extends Event, L extends EventListener<E>> {
+
+    /**
+     * Adds the specified listener.
+     *
+     * @param listener listener to be added
+     */
+    void addListener(L listener);
+
+    /**
+     * Removes the specified listener.
+     *
+     * @param listener listener to be removed
+     */
+    void removeListener(L listener);
+
+
+}