DOCX drawingML export: don't export textboxes of shapes twice

We already had code, so that in case a shape has a textbox, then we
wrote the textbox contents as the shape text, but textboxes were still
exported independently as well.

Build a list of textboxes at the start of the export, and ignore
anything that's on that list during export. In the future, if both RTF
and DOC will support textboxes, then this can be moved to
MSWordExportBase.

Change-Id: I9370b2bbda515ef605281ad398a358b2f24d8f0e