The custom area to add custom widgets.
This bin can be used to add any custom widget to the message area such as a Gtk.ComboBox or Gtk.CheckButton. Note that after adding such widget you will need to call Gtk.Widget.show or Gtk.Widget.show_all on the widget itself for it to appear in the dialog.
If you want to add multiple widgets to this area, create a new container such as Gtk.Grid or Gtk.Box and then add it to the custom bin.
When adding a custom widget to the custom bin, the Granite.MessageDialog.secondary_label's bottom margin will be expanded automatically to compensate for the additional widget in the dialog. Removing the previously added widget will remove the bottom margin.
If you don't want to have any margin between your custom widget and the
Granite.MessageDialog.secondary_label, simply add your custom
widget and then set the Gtk.Label.margin_bottom of
Granite.MessageDialog.secondary_label to 0.