'Return the button_box, which contains the prev and next button
as well the progress label. Creates the instance attributes
self.prev_button and self.next_button.'

def button_box(self):

    button_box = Gtk.Box()
    button_box.override_background_color(Gtk.StateType.NORMAL, Gdk.RGBA(0.1, 0.1, 0.1, 0.1))
    self.prev_button = Gtk.Button.new_with_label('<<')
    self.next_button = Gtk.Button.new_with_label('>>')
    self.prev_button.connect('clicked', self.on_click_move_page, (lambda x: (x - 1)))
    self.next_button.connect('clicked', self.on_click_move_page, (lambda x: (x + 1)))
    button_box.pack_start(self.prev_button, True, True, 0)
    button_box.pack_start(self.progress_label, True, True, 0)
    button_box.pack_start(self.next_button, True, True, 0)
    return button_box
