Support 'id' attribute for menu items contributed by plugins

For menu items in the top menu that are contributed by plugins it is
now optionally possible to specify the 'id' attribute. Having the 'id'
attribute allows GWT UI plugins to manipulate the menu item. E.g. a
GWT plugin could open a dialog box when the user clicks on the menu
item:

  RootPanel rootPanel = RootPanel.get(MENU_ID);
  rootPanel.getElement().removeAttribute("href");
  rootPanel.addDomHandler(new ClickHandler() {
    @Override
    public void onClick(ClickEvent event) {
      dialogBox.center();
      dialogBox.show();
    }
  }, ClickEvent.getType());

Change-Id: Icbe2a5c5af9c126a6dd2373b768cae651e482200

gerrit-gwtui/src/main/java/com/google/gerrit/client/Gerrit.java

@@ -978,6 +978,9 @@ public class Gerrit implements EntryPoint {
   private static void addExtensionLink(final LinkMenuBar m, final TopMenuItem item) {
     final Anchor atag = anchor(item.getName(), item.getUrl());
     atag.setTarget(item.getTarget());
+    if (item.getId() != null) {
+      atag.getElement().setAttribute("id", item.getId());
+    }
     m.add(atag);
   }
 }