If a `GtkMessageDialog` has a non-empty title set, use
that for the accessible name instead of a generic name
indicating the type of the message dialog, as the
window title is generally more informative, if set.
It also better matches the information presented
visually on screen (in the window title, task switchers,...)
and is in line with the handling for non-message-dialog
windows.

This can easily be tested with the "Dialogs and
Message Boxes" sample from gtk3-demo when setting
a title for the message dialog in there like this:

    diff --git a/demos/gtk-demo/dialog.c b/demos/gtk-demo/dialog.c
    index 0eb1c62397..53fb7f8b0e 100644
    --- a/demos/gtk-demo/dialog.c
    +++ b/demos/gtk-demo/dialog.c
    @@ -25,6 +25,8 @@ message_dialog_clicked (GtkButton *button,
                                        "number of times:");
       gtk_message_dialog_format_secondary_text (GTK_MESSAGE_DIALOG (dialog),
                                                 "%d", i);
    +  gtk_window_set_title (GTK_WINDOW (dialog), "Some informative title");
    +
       gtk_dialog_run (GTK_DIALOG (dialog));
       gtk_widget_destroy (dialog);
       i++;