nixos/thunar: init

- Add a module for the thunar file manager, which depends on the xfconf dbus service, and also has a dbus service and a systemd unit.

- Renames the option services.xserver.desktopManager.xfce.thunarPlugins to programs.thunar.plugins.

nixos/modules/module-list.nix

@@ -217,6 +217,7 @@
   ./programs/sway.nix
   ./programs/system-config-printer.nix
   ./programs/thefuck.nix
+  ./programs/thunar.nix
   ./programs/tmux.nix
   ./programs/traceroute.nix
   ./programs/tsm-client.nix