Docx writer: handle SVG images.

This change has several parts:

- In Text.Pandoc.App, if the writer is docx, we fill the media
  bag and attempt to convert any SVG images to PNG, adding these
  to the media bag.  The PNG backups have the same filenames as
  the SVG images, but with an added .png extension.  If the conversion
  cannot be done (e.g. because rsvg-convert is not present),
  a warning is omitted.
- In Text.Pandoc.Writers.Docx, we now use Word 2016's syntax for
  including SVG images. If a PNG fallback is present in the media bag,
  we include a link to that too.

It would be helpful if someone with an old Word version could test
to see that the documents we produce can be opened and viewed with
the PNG fallbacks.  If not, then perhaps we can eliminate the
slightly complex code for producing these fallbacks.

Closes #4058.

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