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)
(cherry picked from commit 013744a5b475e151ca0918565fb369e22dfbc1db)

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

Change-Id: Ic4351fe65cabc12d60214b67c7026a317841f2c7
Reviewed-on: https://gerrit.libreoffice.org/10736
Reviewed-by: Matúš Kukan <matus.kukan@collabora.com>
Tested-by: Matúš Kukan <matus.kukan@collabora.com>