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.

common/mpv/.config/mpv/mpv.conf

@@ -0,0 +1,12 @@
+vo=gpu
+
+profile=gpu-hq
+scale=ewa_lanczossharp
+cscale=ewa_lanczossharp
+video-sync=display-resample
+interpolation
+tscale=oversample
+
+hwdec=auto
+
+sub-font=Arial