- Added support for embeded cover. Demuxer just need to fill psz_arturl meta

- Added support for embeded cover. Demuxer just need to fill psz_arturl meta
field to "attachment://NAME" where NAME is an attachment filename
(returned by DEMUX_GET_ATTACHMENTS)
- Lock input_item_t every time we access p_meta fields (there is still a problem
as playlist code does not always do so...).
- Do not preparse item we have already played.
- Wait that the current playing item is meta parsed before fetching its cover.