LibreOffice is unable to properly import the custom gradient fills
defined in ooxml documents. To prevent data loss, we save the
original gradient fill definition in the shape interopgrabbag and we
write it back to the document on export.

In case the user has changed the fill properties of a shape, the
original fill will be discarded in favor of the new fill.

We have added a new ooxmlexport unit test to test this feature.

We have also added some missing transformations to the methods
getColorTransformationName and getColorTransformationToken in Color
class, and refactored some code in class DrawingML to the method
WriteColor( OUString, Sequence ).

Change-Id: Ie71f89eaa20313561aa9180ea33b76f3fb5e5df6