Set dragged widgets' coordinates based on the move rather than the mouse pointer's coords.

--HG--
branch : dfighter-tools

code/nel/src/gui/widget_manager.cpp

@@ -2633,8 +2633,11 @@ namespace NLGUI
 			else
 			if( draggedElement != NULL )
 			{
-				draggedElement->setXReal( newX );
-				draggedElement->setYReal( newY );
+				sint32 dx = newX - oldX;
+				sint32 dy = newY - oldY;
+
+				draggedElement->setXReal( draggedElement->getXReal() + dx );
+				draggedElement->setYReal( draggedElement->getYReal() + dy );
 				draggedElement->invalidateCoords();
 			}
 		}