Uniquify IDs in SVG-Edit

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.

public/svg-edit/editor/extensions/ext-itex.js

@@ -78,8 +78,6 @@ $(function() {
 				      mrow.appendChild(children[0]);
 				    }
 				    S.sanitizeSvg(math);
-				    //elt.setAttribute('width', math.clientWidth+5);
-				    //elt.setAttribute('height', math.clientHeight+5);
 				    S.call("changed", [elt]);
 				});
 				elt.replaceChild(math, elt.firstChild);