Headerbar linked buttons

sample.glade

<?xml version="1.0" encoding="UTF-8"?>
<!-- Generated with glade 3.22.1 -->
<interface>
  <requires lib="gtk+" version="3.20"/>
  <object class="GtkWindow" id="window">
    <property name="can_focus">False</property>
    <property name="default_width">250</property>
    <property name="default_height">150</property>
    <child type="titlebar">
      <object class="GtkHeaderBar">
        <property name="visible">True</property>
        <property name="can_focus">False</property>
        <property name="show_close_button">True</property>
        <child>
          <object class="GtkButtonBox">
            <property name="visible">True</property>
            <property name="can_focus">False</property>
            <property name="layout_style">start</property>
            <child>
              <object class="GtkButton">
                <property name="label" translatable="yes">Save</property>
                <property name="visible">True</property>
                <property name="can_focus">True</property>
                <property name="receives_default">False</property>
              </object>
              <packing>
                <property name="expand">True</property>
                <property name="fill">True</property>
                <property name="position">0</property>
                <property name="non_homogeneous">True</property>
              </packing>
            </child>
            <child>
              <object class="GtkButton">
                <property name="visible">True</property>
                <property name="can_focus">True</property>
                <property name="receives_default">True</property>
                <child>
                  <object class="GtkImage">
                    <property name="visible">True</property>
                    <property name="can_focus">False</property>
                    <property name="icon_name">document-save-as-symbolic</property>
                  </object>
                </child>
              </object>
              <packing>
                <property name="expand">True</property>
                <property name="fill">True</property>
                <property name="position">1</property>
                <property name="non_homogeneous">True</property>
              </packing>
            </child>
            <style>
              <class name="linked"/>
            </style>
          </object>
        </child>
      </object>
    </child>
  </object>
</interface>