feat: rearrange nix modules

desktop/default.nix

@@ -0,0 +1,9 @@
+{ config, pkgs, ... }:
+
+{
+  imports = [
+    ./x11.nix
+    ./gtk.nix
+    ./gnome.nix
+  ];
+}

desktop/gnome.nix

@@ -0,0 +1,47 @@
+{ config, pkgs, ... }:
+
+with pkgs;
+let
+  gnome-shell-extension-pop-shell = stdenv.mkDerivation rec {
+    pname = "gnome-shell-extension-pop-shell";
+    version = "2020-03-13";
+
+    src = fetchFromGitHub {
+      owner = "pop-os";
+      repo = "shell";
+      rev = "ad4f28cbf185b35d8cb3e8710dabee80737156ce";
+      sha256 = "0a312ilbip6gd4ppff8kjr77ilcbwn3wksf2x8ppj0nhfmrich6n";
+    };
+
+    nativeBuildInputs = [ glib ];
+    buildInputs = [ nodePackages.typescript ];
+
+    # the gschema doesn't seem to be installed properly (see dconf)
+    makeFlags = [ "INSTALLBASE=$(out)/share/gnome-shell/extensions" ];
+  };
+in
+{
+  environment.systemPackages = with pkgs; [
+    gnome3.dconf-editor
+    gnome3.gnome-tweaks
+    gnome3.gnome-shell-extensions # required for user-theme
+    gnomeExtensions.dash-to-panel
+    gnomeExtensions.appindicator
+    gnome-shell-extension-pop-shell
+  ];
+
+  # Enable Gnome 3
+  services.xserver.displayManager.gdm.enable = true;
+  services.xserver.displayManager.gdm.wayland = false;
+  services.xserver.desktopManager.gnome3.enable = true;
+  environment.gnome3.excludePackages = with pkgs; [
+    gnome3.geary
+    gnome3.gnome-weather
+    gnome3.gnome-calendar
+    gnome3.gnome-maps
+    gnome3.gnome-contacts
+    gnome3.gnome-software
+    gnome3.gnome-packagekit
+    gnome3.epiphany
+  ];
+} 

desktop/gtk.nix

@@ -0,0 +1,5 @@
+{ config, pkgs, ... }:
+
+{
+  services.dbus.packages = with pkgs; [ gnome3.dconf ];
+}

desktop/x11.nix

@@ -0,0 +1,8 @@
+{ config, pkgs, ... }:
+
+{
+  # Enable the X11 windowing system.
+  services.xserver.enable = true;
+  services.xserver.layout = "gb";
+  services.xserver.xkbOptions = "eurosign:e";
+}