qml: implement childAt

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

src/private/DragController.cpp

@@ -525,7 +525,7 @@ static DropArea* deepestDropAreaInTopLevel(QWidgetOrQuick *topLevel, QPoint glob
             if (DockRegistry::self()->affinitiesMatch(dt->affinities(), affinities))
                 return dt;
         }
-        w = w->parentWidget();
+        w = KDDockWidgets::parentWidget(w);
     }
 
     return nullptr;

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);

src/private/quick/QWidgetAdapter_quick_p.h

@@ -36,6 +36,12 @@ QT_END_NAMESPACE
 
 namespace KDDockWidgets {
 
+/// @brief Helper since QQuickItem::parentItem() has a different name than QWidget::parentWidget()
+inline QQuickItem *parentWidget(QQuickItem *item)
+{
+    return item ? item->parentItem() : nullptr;
+}
+
 class FloatingWindow;
 class DOCKS_EXPORT QWidgetAdapter : public QQuickItem
 {
@@ -96,7 +102,7 @@ public:
     void setWindowTitle(const QString &) {}
     void setWindowIcon(const QIcon &) {}
     void close();
-    QWidgetAdapter* childAt(QPoint) { return nullptr; }
+    QQuickItem *childAt(QPoint) const;
     void move(int x, int y);
 
     void setParent(QQuickItem*);

src/private/widgets/QWidgetAdapter_widgets_p.h

@@ -27,6 +27,12 @@
 
 namespace KDDockWidgets {
 
+/// @brief Helper since QQuickItem::parentItem() has a different name than QWidget::parentWidget()
+inline QWidget *parentWidget(QWidget *widget)
+{
+    return widget ? widget->parentWidget() : nullptr;
+}
+
 class FloatingWindow;
 
 class DOCKS_EXPORT QWidgetAdapter : public QWidget