Fix PaletteStudio color sorting

[hamoid]

getLuminance() returns a normalized Double.

The comparator used the difference of two normalized luminances and converted the result .toInt() which gave 0 in most cases, which did not sort the colors as expected.

[edwinRNDR]

There is a really nice idiomatic way to do this in Kotlin: colors.sortedBy { getLuminance(it) } and colors.sortedByDescending { getLuminance(it) }

[hamoid]

Yeah much nicer :)