Replace main game menu native GUI by pygame_gui system

[Grimmys]

Implements #20

This is a draft: only the GUI elements from the main menu (including about, leaderboard and load windows) have been replaced by pygame_gui elements.

That was really funny to both deep into the implementation of your project and discover how pygame_gui works, thank you for giving me this possibility!

I'm really impressed by what could be done with pygame_gui, and I personally think that its integration globally improved the UI of your game. But it's only my point of view, and it's why I didn't replaced the core game elements yet (and also because I don't know if you are still working on this project).

It's up to you whether I finish the work or not (you could also finish the work on my branch, it shouldn't be an issue).