Move Menu implementation to OS

[chrisfls]

Menus are unrelated to Core, move its implementation to OS.

[PedroHLC]

But menus in core can be used in setup

[chrisfls]

Alright, but setup should also be moved into OS as well, Core should only handle Landing-> OS swap, but let's delay that change until beta.

This is one of the lowest priority issues we have, let's keep it open just for tracking's sake.