Add softproofing and wide-gamut colour selector to Krita

Most consumer screens can only show a limited amount of colour, this we generally refer to as the sRGB space. Wide gamut editing is about editing values in larger spaces of colour(such as Adobe RGB or the ACES colour space) and emulating how that looks on the screen.

Currently, Krita’s architecture has all the bells and whistles for wide-gamut editing. Two big items are missing: Softproofing and a good internal colour selector dialogue for selecting colours that are outside of the sRGB colour space.