The DragDrop class
The ui.DragDrop class is used to control the events related to drag
& drop events.
When implementing drag & drop in a dialog, the ON DRAG* / ON
DROP dialog control blocks take an ui.DragDrop variable as a parameter to
let you configure and control the drag & drop events. The ui.DragDrop variable
must be declared in the scope of the dialog implementing drag & drop.