code: def _add_separator(self):
      sep = Gtk.Separator()
      sep.set_property("orientation", Gtk.Orientation.VERTICAL)
      self._tool_box.append(sep)
  
imports: import functools
  import io
  import os
  import matplotlib
  from matplotlib import _api, backend_tools, cbook
  from matplotlib.backend_bases import ToolContainerBase, KeyEvent, LocationEvent, MouseEvent, ResizeEvent, CloseEvent
  from gi.repository import Gio, GLib, Gtk, Gdk, GdkPixbuf
  from .None import _backend_gtk
  from ._backend_gtk import _BackendGTK, _FigureCanvasGTK, _FigureManagerGTK, _NavigationToolbar2GTK, TimerGTK
label_desc: Use image node_id 8 for calling the ToolbarGTK4 obj's underlying member method code with example usage: obj._add_separator() without return types