Improve internal visibility support for C++ classes

[ghost]

Internal visibility is currently supported inside LimeIDL but has no effect on C++ generated classes. While the C++ language itself doesn't have support for this sort of visibility, I would propose a solution that separates the public vs the internal generated classes, maybe at the file system + namespace level to make it easier for the library creators to package only the public part of their offering.

[DanielKamkha]

"Separating at the file system level" probably means placing the generated files into different directories. For example, placing "internal" types into an "internal" sub-directory.

"Separating at namespace level" sounds like something that should be done by the IDL file author, not by Gluecodium C++ generator.