Docx writer: make id used in native_numbering predictable.

If the image has the id IMAGEID, then we use the id ref_IMAGEID
for the figure number.  Closes #7551.

This allows one to create a filter that adds a figure number
with figure name, e.g.

     <w:fldSimple w:instr=" REF ref_superfig "><w:r><w:t>Figure X</w:t></w:r></w:fldSimple>

For this to be possible it must be possible to predict the
figure number id from the image id.

If images lack an id, an id of the form `ref_fig1` is used.

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