Fire a specific event when a vote is deleted from a change

Currently, removing a vote causes a "comment added" event to be
fired.

Replace it with a specific event. The event contains the approvals
before and after the removal, the removed vote(s), and the message.

Note that while the current implementation of DeleteVote only allows
to delete a single vote, the event allows for multiple removed votes
in case this is implemented in DeleteVote in future.

Change-Id: I8d62dc61c13d231a47d109f544c472d974031f0c

gerrit-extension-api/src/main/java/com/google/gerrit/extensions/events/VoteDeletedListener.java

@@ -0,0 +1,33 @@
+// Copyright (C) 2016 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.extensions.events;
+
+import com.google.gerrit.extensions.annotations.ExtensionPoint;
+import com.google.gerrit.extensions.common.ApprovalInfo;
+
+import java.util.Map;
+
+/** Notified whenever a vote is removed from a change. */
+@ExtensionPoint
+public interface VoteDeletedListener {
+  interface Event extends RevisionEvent {
+    Map<String, ApprovalInfo> getOldApprovals();
+    Map<String, ApprovalInfo> getApprovals();
+    Map<String, ApprovalInfo> getRemoved();
+    String getMessage();
+  }
+
+  void onVoteDeleted(Event event);
+}