Add better color picker

[garykac]

Copied from https://code.google.com/archive/p/spritely/issues/14

Posted on Feb 28, 2009 by Swift Kangaroo

The RGB sliders are functional, but not very convenient.

It would be nice to have a rect or circle showing the range of colors so that users could just select directly. These color ranges are probably HSV- based, so we might first need a HSV picker.

The colors displayed would need to respect the GBA/NDS color restrictions: RGB 5-bits per color.