update fn:string to return string value (inner text) for elements
regardless of whether they have text content or typed contents.
rename modelObject.elementText to modelObject.textValue
to be in line with XML specifications that use the terms
text value (for text nodes of an element) and string value
(for text nodes of an element and its descendants).  Now
.textValue is a node's own text, and .stringValue is node and
descendant's text.