add swipe_up and Swipe_down events to the GestureDetector

[bobwatcherx]

I'm looking for the right event to do swipe_up and swipe_down but I can't find it in the docs. maybe that can be requested?
like tiktok scrolling for change page

[yunguangli]

on_vertical_drag_start and on_vertical_drag_update can do the trick. use various deltas in the event handler to decide if it is up or down. check the GestureDetector control documentation.