Since we can have several SVG-Edit graphics
on a page, SVG-Edit should assign unique IDs
to elements, and do so in a fashion that survives
re-editing.
To do this, we use a nonce, and record its value in
a custom se:nonce attribute on the <svg> element.
(Is there a better way?).
Also, preserve the custom se:connector attribute for
later editing purposes.
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