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/gtk/default.nix

@@ -0,0 +1,17 @@
+{ config, lib, ... }:
+let
+  cfg = config.my.profiles.gtk;
+in
+{
+  options.my.profiles.gtk = with lib; {
+    enable = mkEnableOption "gtk profile";
+  };
+
+  config = lib.mkIf cfg.enable {
+    # Allow setting GTK configuration using home-manager
+    programs.dconf.enable = true;
+
+    # GTK theme configuration
+    my.home.gtk.enable = true;
+  };
+}