Merge: Generalize MEntity visibility

Proposal to generalize the visibility concept to all the MEntities:
* packages, groups and modules are always public
* classes depend on the signature, classdefs return the class visibility (shortcut)
* properties depend on the signature, propdefs return the property visibility (shortcut)
Other exotic MEntities are always public.

This PR makes the changes in model and update `model_json` as PoC for the gain of the generalization.

Pull-Request: #2122
Reviewed-by: Jean Privat <jean@pryen.org>