Pass an XAdES flag to a couple more functions and adapt to that.

Factor out writeDigestMethod() and writeSignedProperties() from
OOXMLSecExporter::Impl to DocumentSignatureHelper and use them in an
additional place.

Write xd:UnsignedProperties with EncapsulatedX509Certificate. Probably
much more work needed.

Change-Id: I2a0cd1db6dd487b9c7ba256ad29473de3d271cd8