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/.config/helium-browser-flags.conf

@@ -0,0 +1,6 @@
+# Hardware video acceleration (VA-API, AMD radeonsi) on Wayland
+--ozone-platform-hint=auto
+--enable-features=AcceleratedVideoDecodeLinuxGL,AcceleratedVideoDecodeLinuxZeroCopyGL,AcceleratedVideoEncoder
+
+# Darwin Breeze theme, generated by kde/theme/build.py
+--load-extension=~/.local/share/helium-themes/darwinbreeze