Move the xmlsec helper methods to comphelper so that we can use them in cui

Change-Id: If9b10cfff5f5abd6b16e48f043af7959edbb1142
Reviewed-on: https://gerrit.libreoffice.org/63198
Tested-by: Jenkins
Reviewed-by: Samuel Mehrbrodt <Samuel.Mehrbrodt@cib.de>