colorspaces: add Linear Rec2020 RGB
colorin: replace beta rgb with linear rec2020 rgb

both beta rgb and rec2020 rgb are large colorspaces that cover
a large gamut and encompass both srgb and adobergb.

however as far as i'm aware beta rgb has seen little real world use.

rec2020 is the upcoming standard for ultra hdtv, so it's more likely
to be useful in other future use-cases.

incidentally rec2020 also uses the d65 whitepoint like most of our
other embedded colorspaces.
colorspaces: drop betargb as it's no longer being used
colorspaces: rework linear rec709 rgb to v2 profile
colorspaces: don't link trivial trc for adobe rgb
colorspaces: make linear rec2020 rgb available elsewhere
desktop: custom unity launcher background color
usermanual: reflect switch from BetaRGB to linear Rec2020 RGB
streamline color picker calling conventions (request_color_pick)
color picker: minor code optimization
Merge branch 'master' of https://github.com/darktable-org/darktable