Finding related entries

This as also been discussed in #2085

This proposed change make the fields reponsible for
finding relations. This change takes methods that were
in content.publish.php and moves them to the Field class,
preserving all compatibility.

Reponsability is put in the right container and
Field implementors will now be able to defines their own mecanism
in order to create relationships.

The *right* way to solve this would be via composition of a class that would handle the two methods. But since Field is already a big monolitic class, this is the way to do, IMHO.
> Can this function accept the same parameters as `findRelatedEntries`?

This would, in fact, be the ideal case. But, `$field` here is the parent field (instantiated on line 1451) whereas the `$related_field` *is* the child (so it know its own id). In both cases, we only need the child field id In order to make the relationship work. But forcing people to use `$this->get('id')` is a better thing: it avoids possible duplicate information.

If we want to keep the same signature, We could pass the parent field id. Right now, this piece of information is not needed, but it could be in the future.