Merge: objcwrapper: generate valid Nit code for large Objective-C classes

This PR continues the good work of #1661 by improving further the code generator of _objcwrapper_. The main change is the revamp of the code generator module, the detection of unknown types so they are not used in active methods, and improvements to the gammar and preprocessor to accept some GNUStep classes.

The generated code now passes nitpick but there is some problems with the generated Objective-C. Some types are not correctly reproduced. So it is not usable as of yet, but it is not too far.

The main limiting factors at this points are functions pointers used as parameters, the class hierarchy that is not duplicated and the Objective-C FFI not working on GNU/Linux.

Pull-Request: #1664
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>