GTK 3 cleanup: use GtkAboutDialog for the About box.

This is again easier than faffing about doing it manually, and as an
added bonus, we get to put the largest of our icons in the box as a
logo :-)

gtk.c

@@ -2229,6 +2229,21 @@ static void menu_config_event(GtkMenuItem *menuitem, gpointer data)
 static void menu_about_event(GtkMenuItem *menuitem, gpointer data)
 {
     frontend *fe = (frontend *)data;
+
+#if GTK_CHECK_VERSION(3,0,0)
+    extern char *const *const xpm_icons[];
+    extern const int n_xpm_icons;
+    GdkPixbuf *icon = gdk_pixbuf_new_from_xpm_data
+        ((const gchar **)xpm_icons[n_xpm_icons-1]);
+    gtk_show_about_dialog
+        (GTK_WINDOW(fe->window),
+         "program-name", thegame.name,
+         "version", ver,
+         "comments", "Part of Simon Tatham's Portable Puzzle Collection",
+         "logo", icon,
+         (const gchar *)NULL);
+    g_object_unref(G_OBJECT(icon));
+#else
     char titlebuf[256];
     char textbuf[1024];
 
@@ -2239,6 +2254,7 @@ static void menu_about_event(GtkMenuItem *menuitem, gpointer data)
 	    "%.500s", thegame.name, ver);
 
     message_box(fe->window, titlebuf, textbuf, TRUE, MB_OK);
+#endif
 }
 
 static GtkWidget *add_menu_item_with_key(frontend *fe, GtkContainer *cont,