Change trend line UI to add fields: X name and Y name
to change "x" and "f(x)" in equation representation

Next to be done: save it to ODF file

Change-Id: I0680ee1bbfbbb74016ecc858917e10d6790ac63a
Reviewed-on: https://gerrit.libreoffice.org/27069Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Laurent BP <laurent.balland-poirier@laposte.net>