qt5ct: add a nixos module to enable qt5ct

In order to use qt5ct (Qt5 Configuration Tool) to configure Qt5
settings (theme, font, icons, etc.) under DE/WM without Qt
integration, the environment variable QT_QPA_PLATFORMTHEME should be
set to "qt5ct".

It can be done automatically by this module, or by setting the
variable explicitly in the user or in the system configuration.

nixos/modules/module-list.nix

@@ -87,6 +87,7 @@
   ./programs/mtr.nix
   ./programs/nano.nix
   ./programs/oblogout.nix
+  ./programs/qt5ct.nix
   ./programs/screen.nix
   ./programs/shadow.nix
   ./programs/shell.nix

nixos/modules/programs/qt5ct.nix

@@ -0,0 +1,31 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+{
+  meta.maintainers = [ maintainers.romildo ];
+
+  ###### interface
+  options = {
+    programs.qt5ct = {
+      enable = mkOption {
+        default = false;
+        type = types.bool;
+        description = ''
+          Whether to enable the Qt5 Configuration Tool (qt5ct), a
+          program that allows users to configure Qt5 settings (theme,
+          font, icons, etc.) under desktop environments or window
+          manager without Qt integration.
+
+          Official home page: <link xlink:href="https://sourceforge.net/projects/qt5ct/">https://sourceforge.net/projects/qt5ct/</link>
+        '';
+      };
+    };
+  };
+
+  ###### implementation
+  config = mkIf config.programs.qt5ct.enable {
+    environment.variables.QT_QPA_PLATFORMTHEME = "qt5ct";
+    environment.systemPackages = [ pkgs.qt5ct ];
+  };
+}