Use the designer iframe document to create the serializer element and import elements

In ShadowDOM Polyfill, the serializer element was being wrapped, but the import nodes from meta.getImports() are
unwrapped nodes from the iframe document.

Attempting to insert them into the serializer element was throwing exceptions.
Since the ShadowDOM Polyfill cannot wrap elements created from another context, all the elements for serialization shall
be created in the designer's iframe context.

Fixes #20