Consider having IVTable follow visibility configuration

[JeremyKuhne]

IVTable is currently hardcoded to internal. This makes it impossible to utilize by other assemblies when one has configured CsWin32 for public visibility. The fallout of this is that it is impossible to leverage the auto-generation of ComWrappers CCWs in an external project (as done with the IManagedWrapper pattern that WinForms uses).

The requested fix is just to follow whatever the visibility configuration is.

Note that this comes up in my personal project, not WinForms, as WinForms does not publicly expose the generated CsWin32 interop.