DOCX filter: handle spacing's "before" attribute inside tblStylePr

Change-Id: I0617b6d3b9e6d81300f97b0b992081cd10ec2fdd
DOCX filter: handle shd's "themeFillTint" attribute inside tblStylePr

Change-Id: Ib2bc21e0fb5d874ed82ac826eeab209dcb9303eb
Avoid msfilter::util::ConvertColor in DocxTableStyleExport

It's more consistent to do all the any to string conversion on the
import side. Also, when we have the ConvertColor calls in a single
place, audit grab-bag-related calls: those should always enable auto
color conversion.

Change-Id: I39ef74986617ee0ed629607e2a069047879b0422
Avoid local methods in DocxTableStyleExport

We already have DocxTableStyleExport::Impl, use that instead of passing
around the serializer with no good reason.

Change-Id: I8cc47c34201219c4102c5edb21bcc1455da48ff4