Por all usages of QWindop/windowHandle to KDDockWidgets::Window

Removes a lot of asQWidget() code and is also prepared for non-Qt
views.

src/qtquick/Platform_qtquick.h

@@ -24,6 +24,7 @@ public:
     ~Platform_qtquick() override;
     const char *name() const override;
     std::shared_ptr<ViewWrapper> qobjectAsView(QObject *) const override;
+    std::shared_ptr<Window> windowFromQWindow(QWindow *) const override;
 
 private:
     void init();