Add ability to get a relative representation of a PageReference to another one

The goal is similar to what Path#relativize does in Java. It is about constructing a string from two PageReferences P1 and P2 so that the constructed string would represent a relative path from a serialized representation of P1 to a serialization representation of P2.

For example if P1 is wiki:Sandbox/A/B and P2 is wiki:Sandbox/A/D/E, the constructed string would be ../D/E. It could be implemented using the existing Java Path API with some minor changes, in particular when dealing with ../ and ./ representations which should be .. and . instead.