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/theme/.stow-local-ignore

@@ -0,0 +1,8 @@
+^/README.*
+^/\.gitignore
+^/apply\.sh
+^/build\.py
+^/t3
+^/telegram
+^/vscode
+__pycache__