Refactor/dto collections

[michaelKaefer]

Mini refactor. For easier review I did two commits.

Please feel free to decline it, it is really a minor thing..

Notes:

• Contains a minor BC break. If someone uses CollectionInterface as a type hint and for example an object of FilterCollection is passed as an argument it will throw an error. I guess this is an absolute edge case (and if it exists somewhere it can be fixed easily).
• Deprecates CollectionInterface because it is not used nor needed. I checked in Symfony core, here I only found collections, bags, lists etc. that directly implement SPL interfaces.
• Deprecates the static factory methods just because I found no reason why the constructor is private and an extra factory method is needed. (I understand that DTO collections are no services and they do not use service injection in the constructor via the container but in Symfony these collections, bags, lists, etc. all have a public constructor and no static factory method).