Move event attribute classes to a separate package

The Attribute classes are moved from:

  com.google.gerrit.server.events

to the newly created package:

  com.google.gerrit.server.data

The `QueryStats` class is renamed to `QueryStatsAttribute` to be
consistent with the other classes in the package.

This is only a code reorganisation.  No functionality is changed.

Change-Id: I5c125e8d45d8f8e6943021cd90ddba1c364a103e

gerrit-server/src/main/java/com/google/gerrit/server/data/SubmitLabelAttribute.java

@@ -0,0 +1,21 @@
+// Copyright (C) 2012 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.data;
+
+public class SubmitLabelAttribute {
+    public String label;
+    public String status;
+    public AccountAttribute by;
+}