app: make GimpContainerPopup transient to its parent toplevel

so it doesn't appear under the dock it's invoked from. Only
needed on OSX, but the right thing to do on all platforms.

app/widgets/gimpcontainerpopup.c

@@ -478,6 +478,10 @@ gimp_container_popup_show (GimpContainerPopup *popup,
   if (y + requisition.height > rect.y + rect.height)
     y = orig_y - requisition.height;
 
+  gtk_window_set_screen (GTK_WINDOW (popup), screen);
+  gtk_window_set_transient_for (GTK_WINDOW (popup),
+                                GTK_WINDOW (gtk_widget_get_toplevel (widget)));
+
   gtk_window_move (GTK_WINDOW (popup), x, y);
   gtk_widget_show (GTK_WIDGET (popup));
 }