Trendline equation: Following change 27069
https://gerrit.libreoffice.org/27069
Save customized names of X and Y variables in extended ODF

Change-Id: Ie39487866f7671f4468a37f48847038fbd2cec2a
Reviewed-on: https://gerrit.libreoffice.org/27233Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Laurent BP <laurent.balland-poirier@laposte.net>