Flag enums should be typed

[buchanae]

It would be very useful for documentation purposes if flag enums had types.

Currently, the signature for CreateWindow is:

func CreateWindow(title string, x, y, w, h int32, flags uint32) (*Window, error)

...where flags is just plain uint32. If this had a corresponding type WindowFlags uint32, I could easily navigate from the CreateWindow docs to the WindowFlags docs in godoc.org.

If there's no objection, I think I can manage making a PR.

[veeableful]

Sounds good to me @buchanae! I'd very much like to have your PR if it's not too much trouble :joy: