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/spectacle/settings/klipperrc

@@ -0,0 +1,2 @@
+[General]
+SaveImages=true

kde/spectacle/settings/spectaclerc

@@ -0,0 +1,3 @@
+[General]
+autoSaveImage=false
+clipboardGroup=PostScreenshotCopyImage