vlc core: single storage playlist

vlc core: single storage playlist

- Playlist items are stored only once. No category / onelevel separation
- Services Discovery nodes always stay a tree
- The playlist and Media Library are tree or flat depending on variable "playlist-tree".
  It means that if the setting variable says flat, the items that come from playlist
  demuxers are flattened into a single level list.
- The design demanded a different playlist demuxing system. Now playlist demuxers should
  construct a tree of items under an input_item_node_t root and send that using
  input_item_AddSubItemTree. Currently, the old scheme will be retained in modules, because
  there is still some listening to the old event in code (the libvlc media system for example)