Vastly simplify __extract_artwork()

__extract_artwork():
  Remove code duplication, and simplify how artwork information is
  obtained.

  Instead of running through a separate array for each type of artwork
  to be extracted, all the artwork information (block ID, type of
  artwork, and image format), is stored into one single array as a
  single index to be extracted up on each artwork extraction.

__do_extract():
  Removed this function as we now only need to call it once in the
  script as a loop.

Miscellaneous:
  Minor cleanup and comment fixes