Remove Linux Cgo requirement

[gen2brain]

Did you consider using https://github.com/BurntSushi/xgb which is Cgo free/native binding to XCB? I am not sure though, but it should be possible to rewrite with XCB instead of Xlib so Linux implementation can be Cgo free.

[changkun]

It is an interesting option (maybe this fork is more up to date https://github.com/jezek/xgb), and also would be wonderful if we could remove cgo completely. Nevertheless, I think we will consider writing everything ourselves without introducing third dependencies outside golang.org.