Sketchfab embeds

[rsimon]

Add support for 3D previews in the selection box. See data model example, derived from info provided by @ewg118:

<?xml version="1.0" encoding="UTF-8"?>
<rdf:RDF xmlns:dcterms="http://purl.org/dc/terms/" 
         xmlns:foaf="http://xmlns.com/foaf/0.1/"
         xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" 
         xmlns:edm="http://www.europeana.eu/schemas/edm/" 
         xmlns:svcs="http://rdfs.org/sioc/services#">
    <pelagios:AnnotatedThing rdf:about="http://www.example.com/pelagios-objects/1234">
        <dcterms:title>An example object with a 3D representation</dcterms:title>
        <edm:isShownBy rdf:resource="https://sketchfab.com/models/1234"/>
    </pelagios:AnnotatedThing>
    <edm:WebResource rdf:about="https://sketchfab.com/models/1234">
        <!-- Photogrammetry -->
        <dcterms:format rdf:resource="http://vocab.getty.edu/aat/300053580"/>
    </edm:WebResource>
</rdf:RDF>