Remove Boost callable_traits from EventManager

[Behemyth]

To determine the signature of a callable the event manager, EventManager.h specifically, uses Boost callable_traits . This header only library, while nice, uses boost and Soul engine only uses a single definition it provides. Remove this dependency either by waiting till c++20 for Concepts, implementing a class that provides callable signatures, or by depending the callable_traits externally from Boost.