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.h

@@ -457,6 +457,7 @@ public:
 Q_SIGNALS:
 #ifdef KDDOCKWIDGETS_QTWIDGETS
     ///@brief signal emitted when the parent changed
+    /// QtQuick already has QQuickItem::parentChanged(), so add it only for QtWidgets here.
     void parentChanged();
 #endif