Indicate which elements need preserved state for DOM cloning

[AmeliaBR]

As raised by @karip on #90 (Comment here).

The DOM 4 specs for node.cloneNode() method indicates:

Specifications may define cloning steps for all or some nodes. The algorithm is passed copy, node, document, and optionally a clone children flag, as indicated in the clone algorithm.

This allows current state of the node (such as text value in an input element) to also be cloned.

HTML requires that cloned <script> elements keep track of whether they are already started. A note in the DOM spec suggests

"SVG ought to do the same for its script elements, but does not call this out at the moment."

There may be other parts of SVG where special cloning rules are required, such as use elements and animation elements.