Set preference window to modal

data/prefs.ui

 <?xml version="1.0" encoding="UTF-8"?>
 <!--
-  SPDX-FileCopyrightText: 2020-2021 Bruce Cowan <bruce@bcowan.me.uk>
+  SPDX-FileCopyrightText: 2020-2022 Bruce Cowan <bruce@bcowan.me.uk>
 
   SPDX-License-Identifier: GPL-3.0-or-later
 -->
@@ -29,6 +29,7 @@
   </object>
   <template class="RugbyPrefWindow" parent="GtkWindow">
     <property name="title">Preferences</property>
+    <property name="modal">True</property>
     <child>
       <object class="GtkBox">
         <property name="orientation">vertical</property>

src/main.c

 /*
- * SPDX-FileCopyrightText: 2012-2021 Bruce Cowan <bruce@bcowan.me.uk>
+ * SPDX-FileCopyrightText: 2012-2022 Bruce Cowan <bruce@bcowan.me.uk>
  * SPDX-License-Identifier: GPL-3.0-or-later
  */
 
@@ -33,7 +33,7 @@ about_activated (G_GNUC_UNUSED GSimpleAction *simple,
     gtk_show_about_dialog (window,
                            "logo-icon-name", "face-wink",
                            "program-name", "Rugby",
-                           "copyright", "Copyright 2012–2021 Bruce Cowan",
+                           "copyright", "Copyright 2012–2022 Bruce Cowan",
                            "license-type", GTK_LICENSE_GPL_3_0,
                            "authors", authors,
                            "comments", "Rugby scores possiblities program",