Changing time triggers on_change before the widget is closed
When using a Time/DateTime widget which triggers an on_change event, the on_change is sent to the server every time the user modifies some property on the datetimepicker widget (i.e: adding on minute), so for bigger changes a lot of requests are sent to the server.
To match the GTK client behaviour, the on_change should be only triggered when the datetimepicker widget is closed.