preferences: Replace orb slider icons to reduce IPF abuse in the C++

By taking images/misc/orb@2x.png and editing it (in non-trivial ways,
sadly), we get images that are more suitable for the UI than what we had
up to this point, without the result looking upscaled and ugly, and
without requiring a hardcoded IPF full of magic numbers.

I drew the frame for the pressed variation myself using the pressed
slider block thingy from images/buttons/sliders/slider-pressed.png.