Exceptions for attributes on SVGForeignObjectElement

In SVG 1.1 Second Edition, for consistency with attributes on other
interfaces, I’m adding NO_MODIFICATION_ALLOWED_ERR DOMExceptions to the
x/y/width/height attributes on the SVGForiegnObjectElement interface
(for when the node is a readonly node).

-- 
Cameron McCormack ≝ http://mcc.id.au/