Start splitting the various meta module types

Start splitting the various meta module types
* meta reader : read directly from the file, should be made from the demux.
  Examples: taglib, id3tag
* meta fetcher : use web services to get meta, either from the current ones.
  Called only if we don't have the tags after using the meta readers
* art finder : find the URL of the suitable album art. Called according to the
  art policy
* art downloader : used to retrieve the art from special locations. Not needed
  if art is available directly from an URL.
  Examples: APIC ID3 tag reader