[gimp/samm/gimp-2-10-print-focus] Fix wrong monitor on the printing dialog on macOS.

[Alex Samorukov <asamorukov src gnome org>]

commit 320a5766792d992a74113e802e602d6cf35bd3bb
Author: Alex Samorukov <samm net-art cz>
Date:   Tue Jan 14 21:12:00 2020 +0000

    Fix wrong monitor on the printing dialog on macOS.
    
    Add empty parent window, set gimp_window_set_transient and use gtk_widget_show to make sure that window 
signals are working.

 plug-ins/print/print.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)
---
diff --git a/plug-ins/print/print.c b/plug-ins/print/print.c
index 57b83ce569..2e12aee798 100644
--- a/plug-ins/print/print.c
+++ b/plug-ins/print/print.c
@@ -263,7 +263,11 @@ print_image (gint32     image_ID,
   if (interactive)
     {
       gimp_ui_init (PLUG_IN_BINARY, FALSE);
-
+#ifdef GDK_WINDOWING_QUARTZ
+      GtkWidget * printwindow = gtk_window_new(GTK_WINDOW_TOPLEVEL);
+      gimp_window_set_transient (GTK_WINDOW (printwindow));
+      gtk_widget_show(printwindow);
+#endif
       g_signal_connect_swapped (operation, "end-print",
                                 G_CALLBACK (print_settings_save),
                                 &data);