Do not declare API function pointer types using decltype anymore.

Required, they're going to be used in C code too (GLFW)
Merged function_pointer_types.hpp and hooks.hpp into one header.
Renamed hooks.hpp to hooked_functions.h
hooked_functions.h does not cause errors and warnings anymore when used as a prologue header for GLFW
GLFW gets now our prologue header (does not have any effect yet)
Fixed linkage of true API function pointer variables, so that glfw.lib (C code) can find them.