ImplXMLSignatureListener doesn't have to implement
XSignatureCreationResultListener, signatureCreated() is only called at
SignatureCreatorImpl::notifyResultListener(), on an object set by
SignatureCreatorImpl::addSignatureCreationResultListener(). But
addSignatureCreationResultListener() is only called from
XSecController::prepareSignatureToWrite() where the argument is of type
XSecController.

Change-Id: I378bda8b1ffdebb4305bec9eebe093aa2443b4e7