common: move to modules, move some files out, modules: adjust dir structure

modules/nixos/programs/kde.nix

@@ -0,0 +1,23 @@
+{ config, pkgs, ... }:
+
+{
+  services = {
+    displayManager.sddm = {
+      enable = true;
+      wayland = {
+        enable = true;
+        compositor = "kwin";
+      };
+    };
+    desktopManager.plasma6.enable = true;
+  };
+
+  # remove unnecessary packages
+  environment.plasma6.excludePackages = with pkgs.kdePackages; [
+    elisa
+    discover
+    konsole
+    khelpcenter
+    krdp
+  ];
+}