CamelSession: Make forward_to() method asynchronous.

CamelSession: Make forward_to() method asynchronous.

Missed this while converting the rest of Camel to GIO's async pattern,
but came to realize Evolution's forward_to() implementation starts an
asynchronous CamelFolder.append_to() operation and returns TRUE.

So the return value from camel_session_forward_to() does not actually
indicate whether the message was successfully forwarded.  In fact the
caller has no way of knowing!

This calls for an API break.

Split the forward_to() method into synchronous and asynchronous
variations:

  gboolean  (*forward_to_sync)    (CamelSession *session,
                                   CamelFolder *folder,
                                   CamelMimeMessage *message,
                                   const gchar *address,
                                   GCancellable *cancellable,
                                   GError **error);

  void      (*forward_to)         (CamelSession *session,
                                   CamelFolder *folder,
                                   CamelMimeMessage *message,
                                   const gchar *address,
                                   gint io_priority,
                                   GCancellable *cancellable,
                                   GAsyncReadyCallback callback,
                                   gpointer user_data);

  gboolean  (*forward_to_finish)  (CamelSession *session,
                                   GAsyncResult *result,
                                   GError **error);