Dialogs should be of type GtkDialog.
The dialog is now displayed by calling gtk_dialog_run and gtk_window_set_transient_for in the window displaying the dialog. Cleanup is handled using gtk_widget_destroy.
Destructors in the windows using the dialog no longer cleanup the dialog as it is assumed it will be either cleaned up after the response callback, or handled by the destroy_with_parent property set on the dialog itself.
This is a cleanup followup to MR 842.