Glade, how to generate code for template

[Stefan Salewski <mail ssalewski de>]

As an exercise I tried to create the window.ui xml file below with
glade UI editor from scratch. I have already learned that my current
glade version 3.18.3 does not support stack and stack switcher, but
that will change soon with version 3.20. But I was not able to generate
the starting template, that is the line

<template class="ExampleAppWindow" parent="GtkApplicationWindow">

from glade tool. What is the trick?

~/Downloads/gtk+-3.20.1/examples/application10 $ head window.ui 
<?xml version="1.0" encoding="UTF-8"?>
<interface>
  <!-- interface-requires gtk+ 3.8 -->
  <template class="ExampleAppWindow" parent="GtkApplicationWindow">
    <property name="title" translatable="yes">Example Application</property>
    <property name="default-width">600</property>
    <property name="default-height">400</property>
        <child type="titlebar">
          <object class="GtkHeaderBar" id="header">
            <property name="visible">True</property>