Add Property.getXPath method

To get the XPath of a property one must currently call Property.getPath then do string replacement because the path:

• contains an initial /
• expresses list indexes using the old-style for foo/bar[123]/baz instead of the new-style foo/123/baz

Adding a clean Property.getXPath method would simplify this. Note that this method will not add a schema name as prefix if the property's base schema doesn't have a namespace prefix defined.