Allow Lab color targets for new Advanced Color Editor

Loving the new Advanced/Direct Color Editor improvements, but now that one can enter a target RGB value, it would be even better to be able to enter a target LAB value. RGB values are device and profile-dependent, and if you need to match a specific paint or PANTONE color, your most accurate way to do this is using its (device independent, by definition) LAB value.

This would be useful for product and interiors photographers who get asked to match paint/print colours as closely as possible in photos. Getting manufacturer-spec'd (or reliably sensitometrically derived) D50/D65 LAB colors for things is easy and far more reliable than RGB, especially when you're wanting to assign something an accurate color but have the luminosity reliably vary across its texture (e.g. a painted wall, article of clothing, or cabinet).