This class encapsulates a string used as the value of the document-uri() property of a document,
together with a normalized representation of the string used for equality comparisons. The idea
is that on Windows systems, document URIs are compared using case-blind comparison, but the original
case is retained for display purposes.
Normalize the representation of file: URIs to give better equality matching than straight
string comparison. The main purpose is (a) to eliminate the distinction between "file:/" and
"file:///", and (b) to normalize case in the case of Windows filenames: especially the distinction
between "file:/C:" and "file:/c:".