Allow NULL for message_format, as described in the docs and as worked

Allow NULL for message_format, as described in the docs and as worked

Wed Oct 29 15:40:26 2003  Owen Taylor  <otaylor@redhat.com>

        * gtk/gtkmessagedialog.c (gtk_message_dialog_new/
        gtk_message_dialog_new_with_format): Allow NULL
        for message_format, as described in the docs and
        as worked previously. (#125759, Richard Hult)

        * gtk/gtkmessagedialog.c (gtk_message_dialog_new_with_markup):
        Use g_markup_vprintf_escaped() to escape argumetns.

        * gtk/gtkmessagedialog.[ch]: Add gtk_message_dialog_set_markup()
        so there is some way to set literal markup.