Save sidecar when in gimp mode

If we call darktable as a gimp plugin in "file" mode we use the former history stack
but allow further darkroom developments to be done.

These changes are already written to the history database but,
the xmp sidecar should also be updated according to settings found via dt_image_get_xmp_mode()

src/gui/gtk.c

@@ -880,7 +880,10 @@ static gboolean _gui_quit_callback(GtkWidget *widget,
       dt_dev_write_history(darktable.develop);
 
     if(dt_check_gimpmode_ok("file"))
+    {
       darktable.gimp.error = !dt_export_gimp_file(darktable.gimp.imgid);
+      dt_image_write_sidecar_file(darktable.gimp.imgid);
+    }
 
     dt_control_quit();
   }
@@ -905,7 +908,10 @@ static void _quit_callback(dt_action_t *action)
   {
     dt_dev_write_history(darktable.develop);
     if(dt_check_gimpmode_ok("file"))
+    {
       darktable.gimp.error = !dt_export_gimp_file(darktable.gimp.imgid);
+      dt_image_write_sidecar_file(darktable.gimp.imgid);
+    }
   }
 
   dt_control_quit();