qml: implement childAt

Added a namespace-level parentWidget() helper, so it works for
both widgets and quick

src/private/quick/QWidgetAdapter_quick.cpp

@@ -229,6 +229,11 @@ void QWidgetAdapter::close()
     }
 }
 
+QQuickItem *QWidgetAdapter::childAt(QPoint p) const
+{
+    return QQuickItem::childAt(p.x(), p.y());
+}
+
 void QWidgetAdapter::move(int x, int y)
 {
     setX(x);