medialibrary: expose the `publicOnly` query parameter

Allow medialibrary queries to process and list public media.

Public media are a new addition of the medialibrary 0.13 version. With this, the users will have the opportunity to allow a subset of the medialibrary to be exposed on network interfaces (notably the upnp server and the HTTP remote control).

Community discussions around the subject can be found here: !269 (comment 274345)