Simplify ViewFactory by making it receive View parents instead of Controllers

Users don't need to worry about such details, like casting to QWidget.

src/qtquick/views/Stack_qtquick.h

@@ -48,7 +48,7 @@ class DOCKS_EXPORT Stack_qtquick
 
 public:
     explicit Stack_qtquick(Controllers::Stack *controller,
-                           Controllers::Frame *parent = nullptr);
+                           QQuickItem *parent = nullptr);
 
     int numDockWidgets() const override;
     void removeDockWidget(Controllers::DockWidget *) override;