setSmooth feature for Fonts

[dogunbound]

Main SFML has this feature merged: https://github.com/SFML/SFML/pull/1690

I was hoping to see if and when this feature will reach CSFML. I am no C wiz, so someone else will probably have to implement this. I'm using rust-sfml which uses C bindings, so when this feature lands here, I can update rust-sfml.