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:
videolan/vlc!269 (comment 274345)