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-gwtexpui/src/main/java/com/google/gwtexpui/safehtml/client/SafeHtml.java

@@ -15,10 +15,10 @@
 package com.google.gwtexpui.safehtml.client;
 
 import com.google.gwt.core.client.GWT;
+import com.google.gwt.dom.client.Element;
 import com.google.gwt.regexp.shared.MatchResult;
 import com.google.gwt.regexp.shared.RegExp;
 import com.google.gwt.user.client.DOM;
-import com.google.gwt.user.client.Element;
 import com.google.gwt.user.client.ui.HTML;
 import com.google.gwt.user.client.ui.HTMLTable;
 import com.google.gwt.user.client.ui.HasHTML;
@@ -86,13 +86,13 @@ public abstract class SafeHtml
   }
 
   /** @return the existing inner HTML of any element. */
-  public static SafeHtml get(final Element e) {
-    return new SafeHtmlString(DOM.getInnerHTML(e));
+  public static SafeHtml get(Element e) {
+    return new SafeHtmlString(e.getInnerHTML());
   }
 
   /** Set the inner HTML of any element. */
-  public static Element set(final Element e, final SafeHtml str) {
-    DOM.setInnerHTML(e, str.asString());
+  public static Element setInnerHTML(Element e, SafeHtml str) {
+    e.setInnerHTML(str.asString());
     return e;
   }
 
@@ -109,8 +109,10 @@ public abstract class SafeHtml
   }
 
   /** Parse an HTML block and return the first (typically root) element. */
-  public static Element parse(final SafeHtml str) {
-    return DOM.getFirstChild(set(DOM.createDiv(), str));
+  public static com.google.gwt.user.client.Element parse(SafeHtml html) {
+    com.google.gwt.user.client.Element e = DOM.createDiv();
+    setInnerHTML(e, html);
+    return DOM.getFirstChild(e);
   }
 
   /** Convert bare http:// and https:// URLs into <a href> tags. */