core: remove main preparser and main thumbnailer

Supersedes !6129 (closed), on top of !6126 (merged)

This will save 3 threads (from executors) that were systematically created with libvlc. Even though, an interface will trigger the playlist and a preparer creation (2 new threads).

• vlc_playlist_t now create its own vlc_preparser_t.
• All components that need extra parsing need to create their own vlc_preparser_t (or use the input_item_Parse API directly)
  • This will avoid conflicts between the playlist and the network browsing (parsing a folder waiting for the playlist to finish parsing or vice versa)

The thumbnailer and preparser hold by libvlc is temporary and will be removed with !3503