qt: add option platformTheme

This deprecates `useGtkTheme=true` with the intention of replacing it
with the `platformTheme` selection.

modules/misc/qt.nix

@@ -12,27 +12,64 @@ in
 {
   meta.maintainers = [ maintainers.rycee ];
 
+  imports = [
+    (mkChangedOptionModule
+      [ "qt" "useGtkTheme" ]
+      [ "qt" "platformTheme" ]
+      (config:
+        if getAttrFromPath [ "qt" "useGtkTheme" ] config
+        then "gtk"
+        else null))
+  ];
+
   options = {
     qt = {
       enable = mkEnableOption "Qt 4 and 5 configuration";
 
-      useGtkTheme = mkOption {
-        type = types.bool;
-        default = false;
+      platformTheme = mkOption {
+        type = types.nullOr (types.enum [ "gtk" "gnome" ]);
+        default = null;
+        example = "gnome";
+        relatedPackages = [
+          "qgnomeplatform"
+          ["libsForQt5" "qtstyleplugins"]
+        ];
         description = ''
-          Whether Qt 4 and 5 should be set up to use the GTK theme
-          settings.
+          Selects the platform theme to use for Qt applications.</para>
+          <para>The options are
+          <variablelist>
+            <varlistentry>
+              <term><literal>gtk</literal></term>
+              <listitem><para>Use GTK theme with
+                <link xlink:href="https://github.com/qt/qtstyleplugins">qtstyleplugins</link>
+              </para></listitem>
+            </varlistentry>
+            <varlistentry>
+              <term><literal>gnome</literal></term>
+              <listitem><para>Use GNOME theme with
+                <link xlink:href="https://github.com/FedoraQt/QGnomePlatform">qgnomeplatform</link>
+              </para></listitem>
+            </varlistentry>
+          </variablelist>
         '';
       };
     };
   };
 
-  config = mkIf (cfg.enable && cfg.useGtkTheme) {
-    home.sessionVariables.QT_QPA_PLATFORMTHEME = "gtk2";
-    home.packages = [ pkgs.libsForQt5.qtstyleplugins ];
+  config = mkIf (cfg.enable && cfg.platformTheme != null) {
+    home.sessionVariables.QT_QPA_PLATFORMTHEME =
+      if cfg.platformTheme == "gnome" then "gnome" else "gtk2";
+
+    home.packages =
+      if cfg.platformTheme == "gnome"
+      then [ pkgs.qgnomeplatform ]
+      else [ pkgs.libsForQt5.qtstyleplugins ];
+
     xsession.profileExtra =
       "systemctl --user import-environment QT_QPA_PLATFORMTHEME";
 
+    # Enable GTK+ style for Qt4 in either case.
+    # It doesn’t support the platform theme packages.
     home.activation.useGtkThemeInQt4 = dag.entryAfter ["writeBoundary"] ''
       $DRY_RUN_CMD ${pkgs.crudini}/bin/crudini $VERBOSE_ARG \
         --set "${config.xdg.configHome}/Trolltech.conf" Qt style GTK+