fdo#81782 MM: copy most document properties

Mail merge wasn't copying the document properties into the target
document. It also has to copy most document properties into the
working copy, as ConvertFieldsToText is called just there.

Change-Id: I7e07fec712c225f667d960a94f4eb3fd0b83f041
Reviewed-on: https://gerrit.libreoffice.org/10987
Reviewed-by: Björn Michaelsen <bjoern.michaelsen@canonical.com>
Tested-by: Björn Michaelsen <bjoern.michaelsen@canonical.com>