DocxExport::OutputDML: use the right namespace for groupshapes

Change-Id: Idd270e5519587440fe96e73f5cf51851f7680d59
oox: initial ShapeExport::WriteGroupShape

For now, enable it only for DOCX.

Change-Id: I424d8fb3f1c1e7c9a03e7103c91473c3ff85bc77