Removed final from PersistentCollection

I would like to be able to extend PersistentCollection. But it is final.

There is "@todo      Design for inheritance to allow custom implementations?" in PersistentCollection.php.

Ofc it needs more modifications because the instantiation of PersistentCollections is hardcoded in UnitOfWork and ObjectHydrator classes. Creating something like PersistentCollectionFactory or persistentCollectionClass could solve it but there are already lots of dependencies in those classes. I can live with the hardoded state. I can somehow "hack" it so it will always replace PersistentCollection instances with my own which extends PersistentCollection when they are set. It's ugly but it could work..

So... At least removing the final statement helps me and could be a good start. Switching from private to protected for $em and $coll helps me also so i can read them in the descendant class but it can be "hacked" too even if i don't like hacks). Or getEntityManager() and getOriginalCollection() could be added.

If you can't accept this PR, let me know about the idea how "@todo      Design for inheritance to allow custom implementations?" could be implemented and i will do it. If using a factory, class name or whatever.