0.19.0: Generate iconset based on `gnome-colors`

*  fd2c25a actionless chore(readme): add info regarding iconset generation
*  8880b4b actionless feat(gui): add possibility to choose colors for the icon theme, export it; apply global css only for preview area; show both stderr and stdout in the log
*  3bd41c4 actionless feat: add script to generate gnome-colors icon theme
*  16bbbcc actionless chore(gui): update built themes
*  f567c4f actionless chore: add gnome-colors iconset original sources
*  dc65121 actionless fix(gtk3.20): resolve warnings. closes #36