* include/input_ext-intf.h: killed p_es->b_audio, since i_cat already fills
    its purpose.
  * include/input_ext-intf.h: killed p_es->i_type in favour of p_es->i_fourcc.
  * include/video.h: killed p_pic->p_plane->b_margin; now i_visible_pitch is
    always valid.

  * ./src/libvlc.h: replaced --mpeg-adec and --ac3-adec with --codec; see
    vlc --help for more information.

  * include/video.h: removed fourcc values which should _not_ be global
    (thanks fenrir for convincing me).
  * ALL: video output plugins use the VLC_FOURCC macro to build their fourccs.
  * ./plugins/beos/InterfaceWindow.cpp: removed the last remaining call to
    input_ChangeES.

   Things I'm not sure about and need comments:
    - I used "mpga" and "mpgv" for MPEG (1+2) audio and video fourcc values.
    - I also introduced "spu ", "lpcm" and "ac3 ".
    - I removed for instance "MP4_GetCodec" because it wasn't really
      used except for the description string. I hope this change did not
      break anything really important.
    - fenrir, I must have broken a lot of things you wrote. Sorry in advance.