new option: sane.gui.plasma.enable to enable desktop KDE plasma

this is useful for testing applications which are designed for KDE
first.

modules/gui/default.nix

@@ -8,6 +8,7 @@ in
   imports = [
     ./gnome.nix
     ./phosh.nix
+    ./plasma.nix
     ./plasma-mobile.nix
     ./sway.nix
   ];

modules/gui/plasma.nix

@@ -0,0 +1,28 @@
+{ lib, config, ... }:
+
+with lib;
+let
+  cfg = config.sane.gui.plasma;
+in
+{
+  options = {
+    sane.gui.plasma.enable = mkOption {
+      default = false;
+      type = types.bool;
+    };
+  };
+
+  config = mkIf cfg.enable {
+    sane.gui.enable = true;
+
+    # start plasma on boot
+    services.xserver.enable = true;
+    services.xserver.desktopManager.plasma5.enable = true;
+    services.xserver.displayManager.sddm.enable = true;
+
+    # gnome does networking stuff with networkmanager
+    networking.useDHCP = false;
+    networking.networkmanager.enable = true;
+    networking.wireless.enable = lib.mkForce false;
+  };
+}