app, libgimp: disable again cross-process transient setting.

After testing, setting a window as transient to another from another process is
still broken on Windows and it's hard to diagnose without using Windows
directly. Since it's not just broken, but it even hangs the whole process, which
is quite a blocker issue, let's disable again the whole code on Windows.

app/widgets/gimpwidgets-utils.c

@@ -2585,7 +2585,17 @@ gimp_window_set_transient_cb (GtkWidget   *window,
       transient_set = TRUE;
     }
 #endif
-#ifdef GDK_WINDOWING_WIN32
+  /* Cross-process transient-for is broken in gdk/win32. It causes hangs of the
+   * main process and we still don't know why.
+   * If it eventually is fixed to actually work, change this to a run-time check
+   * of GTK+ version. Remember to change also gimp_window_transient_on_mapped()
+   * in libgimp/gimpui.c
+   *
+   * Note: this hanging bug is still happening with GTK+3 as of mid-2023 with
+   * steps described in comment 4 in:
+   * https://bugzilla.gnome.org/show_bug.cgi?id=359538
+   */
+#if 0 && defined (GDK_WINDOWING_WIN32)
   if (! transient_set)
     {
       GdkWindow *parent;

libgimp/gimpui.c

@@ -389,7 +389,10 @@ gimp_window_transient_on_mapped (GtkWidget   *window,
       transient_set = TRUE;
     }
 #endif
-#ifdef GDK_WINDOWING_WIN32
+  /* To know why it is disabled on Win32, see gimp_window_set_transient_cb() in
+   * app/widgets/gimpwidgets-utils.c.
+   */
+#if 0 && defined (GDK_WINDOWING_WIN32)
   if (! transient_set)
     {
       GdkWindow *parent;