fdo#74115 Fix for DOCX GradientFill for Chart Wall

Fixed import and export for chart wall Gradient Fill in DOCX
Added UT for the same.

Conflicts:
	chart2/qa/extras/chart2export.cxx

Change-Id: Ie6caa2b238aeb70f7225145da8c5c78003e73002
small stylistic changes

Change-Id: I31cbdc8e09474252205edb837bfd1a8a7299b52b