Minor refactor of diff code to prepare for unified view

Pulled some classes out of SideBySide2. There will be a lot of
common code between the upcoming Unified2 and SideBySide2.

Fixed a minor bug in skip line rendering. It was still relying
on Side.PARENT. Changed to use DisplaySide.A.

Deleted deprecated PatchSelectBox2.

Change-Id: I87d608d7e6a4330096d65b0fc1b99c1c5160d119

gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/DisplaySide.java

@@ -0,0 +1,20 @@
+// Copyright (C) 2013 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.client.diff;
+
+/** Enum representing the side on a side-by-side view */
+enum DisplaySide {
+  A, B;
+}