[Core][Apps] Making visibility hidden by default

[roigcarlo]

📝 Description
This PR aims to unify the windows and Linux behavior making all symbols in Linux (gcc and llvm based compilers) hidden by default. This will probably help with linking times and linking restrictions (see llvm in win refusing to link targets with more than 65K symbols)

Since exporting template classes with definitions in sources works a bit different (finally i figured out how to make them visible!), some additional decorators will need to be added to several explicit instantiations.

Comments:

• Changes in find_global_nodal_neighbours_for_entities_process.cpp are not strictly necessary, but is better this way (imo).
• @KratosMultiphysics/dem DEM application was failing due to a forward declaration of the SphereElement. Please take a look (0e8e547)
• @KratosMultiphysics/mpi There was an error in the AMGCSolve function as the header was never being used. I made it so it uses the header declaration and the source definition instead of using the definition as a header in the source. Please confirm this was the intended behavior.
• @AlejandroCornejo CL is mostly yours check everything needed is exposed, I really don't know what is used from outside so I treid to make everything visible but maybe I am missing something used in other apps I don't have access to.

[loumalouomega]

You don't need to kodify KRATOS_API for non MSVC ?

[matekelemen]

Oh yes thank you! I got back into compile-heavy development again and was about to tear my nonexistent hair out during linking.

[loumalouomega]

Almost compiled, just Metis is failing apparently

[roigcarlo]

KRATOS_API

Should work as it is rn, I foresaw this when I did the Kratos api, but if some application gets grumpy I will touch it.