qtquick: Fix parentChanged() passing the wrong argument

QQuickItem::parentChanged(QQuickItem *parent) passes the *new*
parent, not *this*

Fixes #243 which is actually invalid, but uncovered this bug.
For #243 you can just use sender(), or connect to a lambda that
captures the dock widget.

src/DockWidgetBase.cpp

@@ -732,10 +732,9 @@ void DockWidgetBase::Private::show()
 void DockWidgetBase::onParentChanged()
 {
 #ifdef KDDOCKWIDGETS_QTWIDGETS
-    // TODO: In v1.4, remove this part and use the signal emitting the arg
     Q_EMIT parentChanged();
 #else
-    Q_EMIT parentChanged(this);
+    Q_EMIT QQuickItem::parentChanged(parentItem());
 #endif
     d->updateToggleAction();
     d->updateFloatAction();