xfce: make xfwm optional

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

@@ -41,6 +41,12 @@ in
           Shell commands executed just before XFCE is started.
         '';
       };
+
+      enableXfwm = mkOption {
+        type = types.bool;
+        default = true;
+        description = "Enable the XFWM (default) window manager.";
+      };
     };
 
   };
@@ -87,7 +93,6 @@ in
         pkgs.xfce.xfce4volumed
         pkgs.xfce.xfce4-screenshooter
         pkgs.xfce.xfconf
-        pkgs.xfce.xfwm4
         # This supplies some "abstract" icons such as
         # "utilities-terminal" and "accessories-text-editor".
         pkgs.gnome3.defaultIconTheme
@@ -99,6 +104,7 @@ in
         pkgs.xfce.xfce4_appfinder
         pkgs.xfce.tumbler       # found via dbus
       ]
+      ++ optional cfg.enableXfwm pkgs.xfce.xfwm4
       ++ optional config.powerManagement.enable pkgs.xfce.xfce4_power_manager
       ++ optional config.networking.networkmanager.enable pkgs.networkmanagerapplet
       ++ optionals (!cfg.noDesktop)