Playlist subitem callback lock inversion

• When a subitem is added to the input item of a playlist item:
• vlc_event_send() takes the event manager event sending lock,
• vlc_event_send() takes the event manager object lock,
• vlc_event_send() releases the event manager object lock,
• the playlist callback takes the playlist lock,
• ...
• When a playlist item is deleted:
• The caller has the playlist lock since playlist_ItemRelease() asserts it,
• vlc_event_detach() takes the event manager event sending lock,
• vlc_event_detach() takes the event manager object lock.

There is a lock inversion between the input item's event manager's event sending lock and the playlist lock.