Make writer-side leaf-node add methods chainable

Currently methods for adding leaf-nodes to output elements, like:

void SMOutputContainer.addCharacters(String str);

do no return anything, unlike add methods that add child elements.

But it would actually make sense to return 'this' from those methods; since that would allow chaining of calls to output adjacent leaf nodes (as well as adding an immediate child element after text).
This seems like a simple but useful improvement.