MainWindow.ui (Código)

<?xml version="1.0" encoding="UTF-8"?>
<!-- Generated with glade 3.38.2 -->
<interface>
    <requires lib="gtk+" version="3.20"/>
    <object class="GtkImage" id="img_go_previous_symbolic">
        <property name="visible">True</property>
        <property name="can-focus">False</property>
        <property name="icon-name">go-previous-symbolic</property>
    </object>
    <object class="GtkImage" id="img_go_previous_symbolic_rtl">
        <property name="visible">True</property>
        <property name="can-focus">False</property>
        <property name="icon-name">go-previous-symbolic-rtl</property>
    </object>
    <object class="GtkImage" id="img_mail_send_receive_symbolic">
        <property name="visible">True</property>
        <property name="can-focus">False</property>
        <property name="icon-name">mail-send-receive-symbolic</property>
    </object>
    <template class="MainWindow" parent="GtkApplicationWindow">
        <property name="can-focus">False</property>
        <property name="border-width">12</property>
        <property name="title" translatable="yes">Gtk.HeaderBar</property>
        <property name="default-width">683</property>
        <property name="default-height">384</property>
        <property name="icon">../../../assets/icons/icon.png</property>
        <child>
            <object class="GtkTextView">
                <property name="visible">True</property>
                <property name="can-focus">True</property>
            </object>
        </child>
        <child type="titlebar">
            <object class="GtkHeaderBar">
                <property name="visible">True</property>
                <property name="can-focus">False</property>
                <property name="title" translatable="yes">Gtk.HeaderBar</property>
                <property name="subtitle" translatable="yes">Gtk.HeaderBar</property>
                <property name="show-close-button">True</property>
                <child>
                    <object class="GtkBox">
                        <property name="visible">True</property>
                        <property name="can-focus">False</property>
                        <child>
                            <object class="GtkButton">
                                <property name="visible">True</property>
                                <property name="can-focus">True</property>
                                <property name="receives-default">True</property>
                                <property name="image">img_go_previous_symbolic</property>
                                <signal name="clicked" handler="_left_arrow" swapped="no"/>
                            </object>
                            <packing>
                                <property name="expand">False</property>
                                <property name="fill">True</property>
                                <property name="position">0</property>
                            </packing>
                        </child>
                        <child>
                            <object class="GtkButton">
                                <property name="visible">True</property>
                                <property name="can-focus">True</property>
                                <property name="receives-default">True</property>
                                <property name="image">img_go_previous_symbolic_rtl</property>
                                <signal name="clicked" handler="_right_arrow" swapped="no"/>
                            </object>
                            <packing>
                                <property name="expand">False</property>
                                <property name="fill">True</property>
                                <property name="position">1</property>
                            </packing>
                        </child>
                    </object>
                </child>
                <child>
                    <object class="GtkButton">
                        <property name="visible">True</property>
                        <property name="can-focus">True</property>
                        <property name="receives-default">True</property>
                        <property name="image">img_mail_send_receive_symbolic</property>
                        <signal name="clicked" handler="_send_mail" swapped="no"/>
                    </object>
                    <packing>
                        <property name="pack-type">end</property>
                        <property name="position">1</property>
                    </packing>
                </child>
            </object>
        </child>
    </template>
</interface>