Build with hidden visibility by default

[aperezdc]

It would be nice to build with -fvisibility=hidden by default, then mark the public functions explicitly as having “default” visibility. For platform plug-ins this will be easy because the only functions that need to be public are already marked with G_MODULE_EXPORT. For libcogcore it will probably be a bit more of work and we may need to define a macro (e.g. COG_API) to mark public symbols in the public API headers.