Support overriding the visibility of the generated accessors

[nesk]

• we sould ignore all private/protected properties
• we should generate accessors for internal/public properties
• a property can be public in a private class, we should be careful with this
• the accessor should have the same visibility than the property
• the user should be able to override the visibility of the whole accessors, to force them being internal for example, to avoid leaks into the public API

[nesk]

This might not be a priority anymore. Maybe we should focus on providing a K2 plugin compiler, like Kopy does. It would be way more versatile.