nixos/pantheon: make it possible to remove core packages

nixos/modules/services/x11/desktop-managers/pantheon.nix

@@ -177,17 +177,28 @@ in
       networking.networkmanager.enable = mkDefault true;
 
       # Global environment
-      environment.systemPackages = with pkgs; [
+      environment.systemPackages = (with pkgs.pantheon; [
+        elementary-session-settings
+        elementary-settings-daemon
+        gala
+        gnome-settings-daemon
+        (switchboard-with-plugs.override {
+          plugs = cfg.extraSwitchboardPlugs;
+        })
+        (wingpanel-with-indicators.override {
+          indicators = cfg.extraWingpanelIndicators;
+        })
+      ]) ++ utils.removePackagesByName ((with pkgs; [
         desktop-file-utils
-        glib
+        glib # for gsettings program
         gnome-menus
         gnome.adwaita-icon-theme
-        gtk3.out
+        gtk3.out # for gtk-launch program
         onboard
         qgnomeplatform
         sound-theme-freedesktop
-        xdg-user-dirs
-      ] ++ (with pkgs.pantheon; [
+        xdg-user-dirs # Update user dirs as described in http://freedesktop.org/wiki/Software/xdg-user-dirs/
+      ]) ++ (with pkgs.pantheon; [
         # Artwork
         elementary-gtk-theme
         elementary-icon-theme
@@ -197,24 +208,14 @@ in
         # Desktop
         elementary-default-settings
         elementary-dock
-        elementary-session-settings
         elementary-shortcut-overlay
-        gala
-        (switchboard-with-plugs.override {
-          plugs = cfg.extraSwitchboardPlugs;
-        })
-        (wingpanel-with-indicators.override {
-          indicators = cfg.extraWingpanelIndicators;
-        })
 
         # Services
         elementary-capnet-assist
         elementary-notifications
-        elementary-settings-daemon
-        gnome-settings-daemon
         pantheon-agent-geoclue2
         pantheon-agent-polkit
-      ]);
+      ])) config.environment.pantheon.excludePackages;
 
       programs.evince.enable = mkDefault true;
       programs.file-roller.enable = mkDefault true;