palette[generator]: Add RGBtoHSV function.

From Skia’s SkColor class.
palette[generator]: Add MedianCutQuantizer class.

Contains ColorNode, ColorBox, ColorDimension (and comparators), and
ColorHistogram.
palette[generator]: Add DominantColorCalculator class.
palette[generator]: Add drag-and-drop image loading.
palette[generator]: Bug fixes and syntax errors.
palette[generator]: Display output colors.
palette[generator]: Add drag-and-drop help message.