added mars, vali's desktop

hosts/vali/mars/i3.nix

@@ -0,0 +1,21 @@
+{ pkgs, lib, config, callPackage, ... }:
+with lib; let
+  cfg = config.myOptions.programs.i3;
+in {
+  options.myOptions.programs.i3.enable = mkEnableOption "i3";
+
+  config = mkIf cfg.enable {
+    services.xserver = {
+      enable = true;
+      xkb.layout =  "de";
+      windowManager = {
+          bspwm.enable = true;
+          i3.enable = true;
+      };
+      displayManager = {
+          gdm.enable = true;
+          defaultSession = "none+i3";
+      };
+    };
+  };
+}