GIF Translator: tiny style fix
GIF Translator: Update default settings

* Palette is set to Optimal which preserves colors of original
* Write transparent gifs is on which preserves transparency
* Transparent auto is on which automatically detects the transparent color
* default transparent color is 255, 255, 255 (aka white) if you select a
  manual transparent color which seems like a more sane default than black.

This allows ShowImage and other programs to save transparent gifs without
removing transparency and without reducing the colors.