Docx reader and writer: support row heads.

Reader: When `w:tblLook` has `w:firstColumn` set (or an equivalent bit
mask), we set row heads = 1 in the AST.

Writer: set `w:firstColumn` in `w:tblLook` when there are row
heads. (Word only allows one, so this is triggered by any number
of row heads > 0.)

Closes #9495.