RoutePointerUpEvent in SlateApplication doesn't check if a drag is occurring for touch events

When SlateApplication is routing a pointer up event, it should check bIsDragDropping and call the widget's OnDrop function. Currently, we check for touch events and call OnTouchEnded without checking if we're dragging.

1. Create a widget that implements NativeOnDrop
2. Create a widget that implements NativeOnMouseButtonDown and NativeOnDragDetected.
3. Using a touch device, drag your draggable widget onto your drop zone widget
4. OnDrop will not be called

How to drag / drop from ScrollBox