Add media parameter to Playlist.Item directory type

Allow complete control from remote of the media to add, now permit a correct Player.Open on directories with mixed items.
Also permit to have metadata loaded when files are added from Player.Open
Forcing a wrong type for a specific playlist return an invalid params.
Now also authorize to queue videos to pictures playlists
Merge pull request #2567 from Tolriq/media_playlistitem

Add media parameter to Playlist.Item directory type