DOCX export: handle anchored TextFrames inside shapes

The exporter already handled TextFrames inside TextFrames, a feature
supported by Writer, and not by Word.

Similarly, if DocxSdrExport::writeDMLAndVMLDrawing() starts a shape,
then anchored TextFrames should be posponed till the end of that shape.

Change-Id: I597a8a46b1cf2348a8ccb9af668ea5afab27541b