Describes the offset of a File URL from its root path as specified by its Content Set.
For e.g., if a file
/home/user/Application1/Project1/src/foo/bar/FooBarj.html is included in the Project by a Content Set
that adds the root directory /home/user/Application1/Project1/src, then the OffsetPath will contain
the path foo/bar/FooBar.html and the root as two separate elements. This is the opposite of what
URLFactory(absoluteURL, relativeSpec) would do.