Upgrade to SFML 3

[ChrisThrasher]

https://github.com/SFML/SFML/releases/tag/3.0.0

With the release of SFML 3 not long ago, I'm interesting in seeing more SFML ecosystem libraries upgrade to version 3. Is SFGUI interested in such an upgrade? If so I'm happy to do the bulk of the work. It shouldn't be too difficult.

EDIT: I see this work has already began in the https://github.com/TankOs/SFGUI/tree/refs/heads/feature/sfml-three branch.

[ChrisThrasher]

master...ChrisThrasher:SFGUI:refs/heads/feature/sfml-three

I finished the upgrade! The changes are vast and need to be double-checked but if this generally looks good then I can submit a PR.

[eXpl0it3r]

Change as been merged with #100