Merge "Add TH element support to SafeHtmlBuilder"

gerrit-gwtexpui/src/main/java/com/google/gwtexpui/safehtml/client/SafeHtmlBuilder.java

@@ -318,6 +318,16 @@ public class SafeHtmlBuilder extends SafeHtml {
     return closeElement("td");
   }
 
+  /** Append "<th>"; attributes may be set if needed */
+  public SafeHtmlBuilder openTh() {
+    return openElement("th");
+  }
+
+  /** Append "</th>" */
+  public SafeHtmlBuilder closeTh() {
+    return closeElement("th");
+  }
+
   /** Append "<div>"; attributes may be set if needed */
   public SafeHtmlBuilder openDiv() {
     return openElement("div");