1.3.0: Flat-Plat GTK+ theme support

610fca5 chore(PKGBUILD, readme): update version; mention Flat-Plat support in readme
2663ca6 feat(gui: theme_model): add ACCENT_BG for flat-plat theme
fea2660 chore(submodule: flat-plat; PKGBUILD): use upstream Flat-Plat
1ab5c51 feat(gui: preview; submodule: flat-plat): add specific overrides for sel and header FGs
06e6f81 chore(gnome-colors; PKGBUILD): bundle gnome-colors-common together with exported gnome-colors icon theme
42c949b chore(README, PKGBUILD): mention new dependencies
d310504 refactor(gui: preview): isolate oomox-gtk-theme - specific preview overrides
5e16f10 feat(gui: theme_model; submodules: flat-plat): add option to not generate dark theme variant for GTK+3
71f30b8 chore(colors: retro): add 'c64' theme
b242bc2 feat(gui: color_list, export, preview, theme_model, css): add Flat-Plat theme style
1548d8c doc(CREDITS): add new oomox contributors