Fixed positioning

Hi,

For `proton-native`, I want to incorporate Yoga Layout, which I can do on the JS side without adding extra dependencies here, but I would need a way to fixed position place widgets using x and y coordinates. I've been playing around with GTK, and I got it working here: https://github.com/kusti8/libui, but I don't know how to do that for Windows or for Mac. This would be more of a substitute for Grid and make it a lot easier to use and more stable.

Thanks,
Gustav