Differenciate when gradient is defined in the text element or in its parents

Add unit tests

Change-Id: I85b09f72fbb1a3c8d2fa7350b4396b5bf924277f
Reviewed-on: https://gerrit.libreoffice.org/22131Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Jochen Nitschke <j.nitschke+logerrit@ok.de>
Reviewed-by: Xisco Faulí <anistenis@gmail.com>