add go.mod and refactor code

[GeertJohan]

Nice clean up, it's been a long time since I wrote this code, I barely recognize it, haha.

One thing should probably be changed though. setSizes is a method on sizes, but also returns the same sizes.. That's kind of strange. I'd prefer it to not return the sizes object. Then use it like this:

curSizes.setSizes(screen.Size())
sizesUpdateCh <- curSizes

[robatipoor]

Yes you are right please review again commit

[GeertJohan]

Looks good, thanks!