Update raygui to 4.0

[Merlin04]

Raygui 4.0 has many useful new features that the current version of raygui used by this repo doesn't have: https://github.com/raysan5/raygui/releases/tag/4.0

I'm not sure exactly how much work it would take to upgrade all the way from 3.0 to 4.0, but I imagine it would be pretty significant? Possibly just going to 3.6 could be a good middle ground if 4.0's changes are too big of an overhaul (I'm not familiar with raylib outside of a small amount of time spent using this library, so this may or may not be the case!)