drag-and-drop: add const prefix to member data of DragData
drag-and-drop: add const prefix to member data of DragData

Change-Id: I5988f5200acab6c08f529ae4a506576ce65bff23

dalihub/dali-adaptor,dalihub/dali-adaptor,dalihub/dali-adaptor,dalihub/dali-adaptor,dalihub/dali-adaptor