refactor: group stow packages into common, mac and kde

Split the packages by platform so each app stays an opt-in Stow package
under common/, mac/ or kde/. KDE gets its own Fastfetch, Helium and
Spectacle packages, and darwin-breeze becomes kde/theme. The READMEs
document stowing a whole group as well as individual packages.

kde/helium/README.md

@@ -0,0 +1,11 @@
+# Helium
+
+Browser flags and the generated Darwin Breeze theme. From the repository root:
+
+```sh
+stow -d kde -t "$HOME" helium
+```
+
+The theme files are included, so the KDE `theme` package does not need to be
+stowed to use Helium's theme. To regenerate its colors from the shared color
+scheme, run `python3 kde/theme/build.py` and restart Helium.