QQuickWebEngineView: drag 'n drop does not update the mouse cursor

When dragging an item within a web page the mouse cursor is not updated.

For example on http://gaearon.github.io/react-dnd/examples-chessboard-tutorial-app.html the mouse cursor should be Qt::ForbiddenCursor on forbidden chess board fields, but it always is Qt::DragMoveCursor.

Even when QQuickWebEngineView::dragMoveEvent ignores the event properly (see https://codereview.qt-project.org/#/c/182641/1) the cursor is always Qt::DragMoveCursor.

This is due to code in QQuickWindowPrivate::deliverDragEvent which creates a "translated" copy of the event object but ignores its accepted flag after being handled by QQuickItem::dragMoveEvent.