Rewrite/redesign raygui

[gen2brain]

Raygui was rewritten in Go a long time ago. In the meantime, the C version is rewritten and new features are added. Some possible actions now are to rewrite again in Go or maybe use modernc ccgo and work with that. The other possibility is to create C bindings, which would require a lot of code duplications and will be more difficult to maintain as noted in PR https://github.com/gen2brain/raylib-go/pull/183.