List playlist folders

A playlist folder organizes playlists, smartlists and collections into a private tree. The tree belongs to you alone and is never visible to another user, so filing someone else's public playlist changes nothing for them. Other users' top-level public folders are appended after it; hidden marks the ones you have chosen to hide from your own root, without dropping them from the list. The whole tree is returned as a flat list; rebuild the hierarchy from each folder's parent, where 0 is the root.