CLN: small readability fixes in window.pyx
Method dispatch by name is more in line with the rest of the file.
there's Also already `Window` and `Rolling` classes, so `roll_window` doesn't make clear where it fits in.

I'd also like to rename `Window` to `WeightedWindow`. But that class is returned to the user from `df.rolling` so not sure if that's considered a breaking change. 