Support regression curve name in OOXML (import & export).

Change-Id: I847e0df4f160e4b5078961a0e77c1e1e3fff9bd4
Reviewed-on: https://gerrit.libreoffice.org/6875
Reviewed-by: Björn Michaelsen <bjoern.michaelsen@canonical.com>
Reviewed-by: Thorsten Behrens <thb@documentfoundation.org>
Tested-by: Thorsten Behrens <thb@documentfoundation.org>