include/vlc_events.h: Opt for the vlc_object_name_function_name naming scheme.

* Protect input item's meta through setters and getters. That allows tracking of input item meta info changes.
* vlc_meta_t gains a table for accessing its non-extra meta.

Runs make update-po

modules/meta_fetcher/luameta.c: Fix file name in header, and add Antoine in authors list as it is a derivative.