Docx reader: handle table column relative widths

These should be easy to extract from the
```xml
      <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.
