Add .gpl palette for popular open source graphics editors
Popular open source graphics editors use common standard when it comes to palettes, .gpl
.
In terms of best compatibility with Inkscape, it's best to structure shades and tints in descending order of lightness in a colour group, because they are shown as a long stripe under the editor:
Documentation: https://docs.gimp.org/en/gimp-concepts-palettes.html
Example: https://gitlab.gnome.org/Teams/Design/HIG-app-icons/blob/master/GNOME%20HIG.gpl