Change how darken/lighten and saturate/desaturate works

[Ogeon]

The current implementation of lighten and darken uses absolute values (like self.lightness + amount), but that gives wildly different results between color spaces and isn't usually desired result. It may be more useful if they changed relative to the extremes, like self.lightness + (1.0 - self.lightness) * amount. The current behavior is on the other hand also useful.

Saturate and desaturate are currently implemented as self.saturation * (1.0 + factor) and self.saturation * (1.0 - factor), but should be changed to have the same behavior as lighten/darken.

I'm thinking it would be nice to give both options, but make the relative mode the "default". So there could be lighten and lighten_absolute or lighten_fixed and so on. The relative modes could be default implemented using the absolute modes.