Add Path::relativePath and Path::isParentOf

While I would have liked to reuse KUrl::relativePath I found its
return values very odd. I got e.g. for '/foo' and '/foo/bar' a
relative path './bar' while I would like to have 'bar'. KUrl would
even return './' for relativeUrl when an argument is invalid - odd.

Anyways, the custom implementations of both should also be faster.