iOS: add a temporarily exposed function that calls CallbackFlushHandler::Invoke in order to manually flush the callback queue. To be reverted once the callback problem is solved.

Change-Id: I95273b8a01eec62f639edd6767f69e02a294d8aa
Reviewed-on: https://gerrit.libreoffice.org/48101Reviewed-by: jan iversen <jani@libreoffice.org>
Tested-by: jan iversen <jani@libreoffice.org>