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.

lib/sanitizer.rb

@@ -60,7 +60,7 @@ module Sanitizer
        patternContentUnits patternTransform patternUnits points
        preserveAspectRatio primitiveUnits r refX refY repeatCount repeatDur
        requiredExtensions requiredFeatures restart rotate rx ry se:connector
-       slope spacing
+       se:nonce slope spacing
        startOffset stdDeviation stemh stemv stop-color stop-opacity
        strikethrough-position strikethrough-thickness stroke stroke-dasharray
        stroke-dashoffset stroke-linecap stroke-linejoin stroke-miterlimit