Subclasses should override this method to get notified of the start of a new drag sequence.
Unlike the OnMouseDrag method, this method will not get called until after the MinDragStartDistance has been reached.
Notes to Inheritors: Overriding methods must still call base.OnStartDrag()
for correct behavior.
PDragSequenceEventHandler Class | UMD.HCIL.Piccolo.Event Namespace