Replace includes with forward declaration(s)

This feature allows to have Includator look for include directives which can be replaced with a forward declaration of a class or function.

The following code

//foo.h
#include "A.h" 

void foo(A* pA);
//A.h
struct A { ... };

can e.g. be replaced by this code
//foo.h
struct A;

void foo(A* pA);
//A.h
struct A { ... };

this reduces physical dependencies a lot and can hence lower compile time significantly.

Notice: This feature is still under development and not yet fully functional. Forward declaration are currently inserted into the parent scope of a file while namespaces that enclose the forward declaration are ignored.