Unify SideBySide2 header widgets

Combine the two header widgets into a single panel and compact
the entire display to take up less overall screen space. The
header is now only a single line of text.

The format looks like:

  [x] path/of/FILE                                   < ^ >

The reviewed checkbox is on the left next to the file name.

The path uses plain text for the directory part and BOLD for the
file name itself. This makes it easier to spot the specific file
name being reviewed.

The navigation arrows are now clustered on the right using a
small icon for each. This saves a lot of screen space.

Change-Id: Ibd6b7e1022bfe3d6e97638df1fc94f492dae9af1

gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/Header.ui.xml

@@ -0,0 +1,55 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+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.
+-->
+<ui:UiBinder
+    xmlns:ui='urn:ui:com.google.gwt.uibinder'
+    xmlns:g='urn:import:com.google.gwt.user.client.ui'
+    xmlns:x='urn:import:com.google.gerrit.client.ui'>
+  <ui:style>
+  .header {
+    position: relative;
+  }
+  .reviewed input {
+    margin: 0;
+    padding: 0;
+    vertical-align: bottom;
+  }
+  .path {
+  }
+  .navigation {
+    position: absolute;
+    top: 0;
+    right: 15px;
+  }
+  </ui:style>
+  <g:HTMLPanel styleName='{style.header}'>
+    <g:CheckBox ui:field='reviewed'
+        styleName='{style.reviewed}'
+        title='Mark file as reviewed (Shortcut: r)'>
+      <ui:attribute name='title'/>
+    </g:CheckBox>
+    <span ui:field='filePath' class='{style.path}'/>
+
+    <div class='{style.navigation}'>
+      <x:InlineHyperlink ui:field='prev'>&#x21e6;</x:InlineHyperlink>
+      <x:InlineHyperlink ui:field='up' title='Up to change (Shortcut: u)'>
+        <ui:attribute name='title'/>
+        &#x21e7;
+      </x:InlineHyperlink>
+      <x:InlineHyperlink ui:field='next'>&#x21e8;</x:InlineHyperlink>
+    </div>
+  </g:HTMLPanel>
+</ui:UiBinder>