Showdown window.ui file with open button in headerbar

showdown-window.ui

<?xml version="1.0" encoding="UTF-8"?>
<!-- Generated with glade 3.22.1 -->
<interface>
  <requires lib="gtk+" version="3.14"/>
  <template class="ShowdownWindow" parent="GtkApplicationWindow">
    <property name="width_request">640</property>
    <property name="height_request">480</property>
    <property name="can_focus">False</property>
    <property name="title" translatable="yes">Markdown Viewer</property>
    <property name="default_width">1100</property>
    <property name="default_height">700</property>
    <property name="icon_name">showdown</property>
    <child type="titlebar">
      <object class="GtkHeaderBar" id="header">
        <property name="visible">True</property>
        <property name="can_focus">False</property>
        <property name="title" translatable="yes">Markdown Viewer</property>
        <property name="show_close_button">True</property>
        <child>
          <object class="GtkButton" id="open_button">
            <property name="label">gtk-open</property>
            <property name="visible">True</property>
            <property name="can_focus">False</property>
            <property name="receives_default">True</property>
            <property name="use_stock">True</property>
          </object>
        </child>
        <child>
          <object class="GtkToggleButton" id="search_button">
            <property name="visible">True</property>
            <property name="can_focus">False</property>
            <property name="receives_default">False</property>
            <property name="tooltip_text" translatable="yes">Search</property>
            <child>
              <object class="GtkImage">
                <property name="visible">True</property>
                <property name="can_focus">False</property>
                <property name="icon_name">edit-find-symbolic</property>
                <property name="icon_size">1</property>
              </object>
            </child>
            <accelerator key="f" signal="clicked" modifiers="GDK_CONTROL_MASK"/>
          </object>
          <packing>
            <property name="pack_type">end</property>
            <property name="position">1</property>
          </packing>
        </child>
        <child>
          <object class="GtkMenuButton" id="menu_button">
            <property name="visible">True</property>
            <property name="can_focus">False</property>
            <property name="receives_default">False</property>
            <child>
              <object class="GtkImage">
                <property name="visible">True</property>
                <property name="can_focus">False</property>
                <property name="icon_name">open-menu-symbolic</property>
                <property name="icon_size">1</property>
              </object>
            </child>
            <accelerator key="F10" signal="clicked"/>
          </object>
          <packing>
            <property name="pack_type">end</property>
            <property name="position">1</property>
          </packing>
        </child>
      </object>
    </child>
    <child>
      <object class="GtkGrid" id="grid">
        <property name="visible">True</property>
        <property name="can_focus">False</property>
        <property name="orientation">vertical</property>
        <child>
          <object class="GtkSearchBar" id="search_bar">
            <property name="visible">True</property>
            <property name="can_focus">False</property>
            <child>
              <object class="GtkSearchEntry" id="search_entry">
                <property name="visible">True</property>
                <property name="can_focus">False</property>
                <property name="width_chars">42</property>
                <signal name="activate" handler="search_entry_activate" swapped="no"/>
                <signal name="search-changed" handler="search_entry_changed" swapped="no"/>
              </object>
            </child>
          </object>
          <packing>
            <property name="left_attach">0</property>
            <property name="top_attach">0</property>
          </packing>
        </child>
      </object>
    </child>
  </template>
</interface>