Add stream event and hook for hashtag changes

The stream event is fired and the hook is executed when hashtags
are added or removed.

Change-Id: Ifa66db4d5ea3d5aecd9bf56830cd50688cc6af29

gerrit-server/src/main/java/com/google/gerrit/server/events/HashtagsChangedEvent.java

@@ -0,0 +1,27 @@
+// Copyright (C) 2014 The Android Open Source Project
+//
+// 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 com.google.gerrit.server.events;
+
+import com.google.gerrit.server.data.AccountAttribute;
+import com.google.gerrit.server.data.ChangeAttribute;
+
+public class HashtagsChangedEvent extends ChangeEvent {
+  public final String type = "hashtags-edited";
+  public ChangeAttribute change;
+  public AccountAttribute editor;
+  public String[] added;
+  public String[] removed;
+  public String[] hashtags;
+}