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/tray-icons/.local/share/ngosen-breeze-icons/ngosen-breeze-icons.hook

@@ -0,0 +1,10 @@
+[Trigger]
+Operation = Install
+Operation = Upgrade
+Type = Package
+Target = fcitx5-ngosen
+
+[Action]
+Description = Applying Breeze-style tray icons for Ngó Sen...
+When = PostTransaction
+Exec = /usr/local/share/ngosen-breeze-icons/apply.sh