* takes XML-Sec signature template
* calculates digest for all added reference streams
* produces signature value over canonicalized signature template
* writes same as detached signature into template, streams that out

Change-Id: I303e080dc25c440175fe1ae897f03cf356d5d0f2
Reviewed-on: https://gerrit.libreoffice.org/36106Reviewed-by: Siegmund Gorr <siegmund.gorr@cib.de>
Reviewed-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>
Tested-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>