DOCX drawingML shape import: handle wp:align for wps:txbx positioning

I.e. previously only absolutely positioned textboxes were handled, with
this, centered textboxes are fine as well. Given that the drawingml
picture import code already supported this, refactor common code to a
common method.

Change-Id: I56e0dbfa0ffc7e27a70a0e8fb8477681389a51cf