and restore it from a file chosen by the user.

- win32/*:

  - implementation of the previous feature in the win32 interface
>   -> what the other interfaces need to do to support it is simply to
>      call the two following functions:
>       int playlist_LoadFile(playlist_t *p_playlist,const char *psz_filename);
>       int playlist_SaveFile(playlist_t *p_playlist,const char *psz_filename);
>      where psz_filename is the name of the destination/source file.

  - ability to open several files at a time