Improve palette reduction algorithm

[max-vogler]

To reduce the amount of colors of non-pixelart images, we currently pick the n most common colors and replace all other colors with the closest selected color. There are three big areas for optimization:

• Proposing a target color set (e.g. with k-means?)
• Improve the distance function, which currently only calculates the sum of differences of RGB values.
• Reducing the palette of an image to the target color set