nixos: profiles: move from top-level

My profiles are actually just "special" NixOS modules in that they
orchestrate settings that usually span the NixOS/home-manager boundary,
or otherwise set up configurations from multiple modules at once.

modules/nixos/profiles/x/default.nix

@@ -0,0 +1,23 @@
+{ config, lib, pkgs, ... }:
+let
+  cfg = config.my.profiles.x;
+in
+{
+  options.my.profiles.x = with lib; {
+    enable = mkEnableOption "X profile";
+  };
+
+  config = lib.mkIf cfg.enable {
+    # Enable the X11 windowing system.
+    services.xserver.enable = true;
+    # Nice wallpaper
+    services.xserver.displayManager.lightdm.background =
+      let
+        wallpapers = "${pkgs.plasma5Packages.plasma-workspace-wallpapers}/share/wallpapers";
+      in
+      "${wallpapers}/summer_1am/contents/images/2560x1600.jpg";
+
+    # X configuration
+    my.home.x.enable = true;
+  };
+}