Slate drag/drop detection does not work with multiple pointers

The drag delta detection in SlateApplication does not ensure that the two pointer events it is comparing are coming from the same pointer. This causes it to think dragging should begin on a widget before the pointer interacting with the widget has moved at all.

1. Create a draggable widget
2. Drag the widget. Note that the cursor must move a certain distance before dragging begins
3. Add an empty actor and attach a Widget Interaction Component
4. Drag the widget again. Note that the drag is now triggered immediately