activex: allow to parse item and retreive information before playing

  VLC ActiveX API doesn't follow libvlc API and doesn't expose Media to the user
  so preparse is made by allowing to preparse a specific item from the playlist.

  Informations about the media can be retreive using the same API (video.width, ...),
  throught a fallback when the MediaPlayer isn't playing.

  parse will only allow parsing the first item of the playlist, as other item, and information
  about them, are only accessible by playing them.

Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>