SwTextBoxHelper::findTextBox: add an UNO version

Export filters don't have access to SwXShape, and it's probably good to
have that way.

Change-Id: I039bafec10d2fde7c7acdcfad5b07068a2a519f1
DOCX drawingML export: if shape has textbox, export its contents as shape text

Change-Id: I54a51189e1c595841b8b02f3b4436da4a29f1dac
DOCX drawingML export: avoid duplicated <wps:spPr> for textboxes

It's valid to have multiple ones from these, but the first one was
generated from the shape, the second from the textbox. Given that we use
the textbox only as a container, don't write its properties, it's not
necessary and may cause problems.

Change-Id: Id1f8bb92c587fe8c1437e412b41510803595dd80