Overhaul color-schemes for v5X

Obsoletes DFHack#1599.
Cuts out an overwhelming majority of the old script in favour of dfhack-native implementations and a simpler interface.
Uses Milo Christiansen's implementation of parsing the color files, because we think it's more elegant.

Removes the in-file help text in favour of the .rst documentation.
Removes registration as a concept.
Stores the default scheme in prefs/color.txt instead of a dedicated file.

Anything that uses this script expecting it to work the same as before will break. But then, v5x broke it anyway, so i think that's fine.
Only really works with the ASCII interface.
We haven't written a changelog entry yet.

If this looks good to you [barring a changelog entry, and other changes], we'll move on to season-palette.

Author: Moth-Tolias