iconSize = Gtk.IconSize.LARGE_TOOLBAR
openIcon = Gtk.Image.new_from_icon_name('document-open', iconSize)
self.fileOpen = Gtk.ToolButton.new(openIcon, 'Open')
self.fileOpen.connect('clicked', self.on_FileOpenStandard_activate)
toolbar.insert(self.fileOpen, -1)
What is the non-deprecated way to display a stock icon in GTK3?