bnc#886540: Default chart background for pptx docs should be transparent.

Charts in docx and xlsx OTOH use solid white as the default fill style.

(cherry picked from commit 4a8f2431718f99de6fd9ee3461d703d007261c03)

Conflicts:
	oox/source/drawingml/chart/chartspaceconverter.cxx
	oox/source/ppt/pptimport.cxx

Change-Id: Ic4351fe65cabc12d60214b67c7026a317841f2c7
Reviewed-on: https://gerrit.libreoffice.org/10737
Reviewed-by: Matúš Kukan <matus.kukan@collabora.com>
Tested-by: Matúš Kukan <matus.kukan@collabora.com>