* First simple XCF thumbnailing. Convert from ImageMagick has buggy

* First simple XCF thumbnailing.  Convert from ImageMagick has buggy
  support for XCF, though, so one of the solutions at
  http://stackoverflow.com/questions/5794640/how-to-convert-xcf-to-png-using-gimp-from-the-command-line
  is probably better.

* File::maybeDoTransform() since (at least) r106752 neglects what
  BitmapHandler::getThumbType() returns, so add support for that.

* Add $wgImageMagickIdentifyCommand to avoid writing a parser for XCF
  metadata.