Docx reader: handle table column relative widths

[jgm]

These should be easy to extract from the

      <w:tblGrid>
        <w:gridCol w:w="2337"/>
        <w:gridCol w:w="2337"/>
        <w:gridCol w:w="2338"/>
        <w:gridCol w:w="2338"/>
      </w:tblGrid>

element which is a direct child of w:tbl.

The only complication is that we don't know the page width, so we may need to guess a bit.

[jgm]

Also, we should have some mechanism for determining when a table is "simple" and should have no embedded width information. I'm not quite sure how to do that in a docx context.