nixos/mate: Add extraPanelApplets, extraCajaExtensions option

Hopefully this is more user-friendly.

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

···
       };
 
       debug = mkEnableOption (lib.mdDoc "mate-session debug messages");
+
+      extraPanelApplets = mkOption {
+        default = [ ];
+        example = literalExpression "with pkgs.mate; [ mate-applets ]";
+        type = types.listOf types.package;
+        description = lib.mdDoc "Extra applets to add to mate-panel.";
+      };
+
+      extraCajaExtensions = mkOption {
+        default = [ ];
+        example = lib.literalExpression "with pkgs.mate; [ caja-extensions ]";
+        type = types.listOf types.package;
+        description = lib.mdDoc "Extra extensions to add to caja.";
+      };
     };
 
     environment.mate.excludePackages = mkOption {
···
       (pkgs.mate.basePackages ++
       pkgs.mate.extraPackages ++
       [
+        (pkgs.mate.caja-with-extensions.override {
+          extensions = cfg.extraCajaExtensions;
+        })
+        (pkgs.mate.mate-panel-with-applets.override {
+          applets = cfg.extraPanelApplets;
+        })
         pkgs.desktop-file-utils
         pkgs.glib
         pkgs.gtk3.out

pkgs/desktops/mate/default.nix

···
     pluma = callPackage ./pluma { };
     python-caja = callPackage ./python-caja { };
 
+    # Caja and mate-panel are managed in NixOS module.
     basePackages = [
-      caja-with-extensions
       libmatekbd
       libmatemixer
       libmateweather
···
       mate-icon-theme
       mate-menus
       mate-notification-daemon
-      mate-panel-with-applets
       mate-polkit
       mate-session-manager
       mate-settings-daemon