A lot of code online does manual forward declarations of our classes, this is bad for us since it makes changing implementation details unnecessarily harder e.g a simple change from class to struct can cause linker errors on msvc, having official ways to forward declare our types prevent these issues and allows stuff like adding inline namespaces for versioning purposes.

Also having these headers removes our manual forward declarations as well.

These are some searches of users manually doing it this alone shows 8k uses

https://github.com/search?q=%2Fnamespace%20sf%5Cs*%5C%7B%5Cs*%5Cn%5Cs*(class%7Cstruct)%2F%20NOT%20repo%3Asfml%2FSFML%20language%3AC%2B%2B&type=code

I am making a pr to have a discussion.