Add support for Swatchbooker's SBZ colour palette format

Operating System: (if feature is platform specific)

The file format of Swatchbooker (SBZ) is being used to store colour palettes. Currently, apart from Swatchbooker, Krita and Scribus support this XML format. Inkscape is planning support for it once they have rewritten the relevant parts of their code. Apart from being open and fully documented (http://www.selapa.net/swatchbooker/) it would provide GIMP users the opportunity to use freely available (CC licence) and mostly commercial colour palettes in LAB values from freieFarbe e.V. (https://www.freiefarbe.de/thema-farbe/software/). The palettes from commercial vendors aren't identical to their "offcial" colour values but have been created with their consent using colorimetric measuring tools. Of course it would require enabling the use of LAB colours for palettes.

Use cases

Currently GIMP only supports palettes in its old and outdated GPL format, using (s)RGB values. Supporting SBZ would give users access to a wide range of real-world colour systems with printed colour references in LAB.