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

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

src/qtcommon/Platform_qt.h

@@ -25,6 +25,9 @@ public:
     ~Platform_qt() override;
 
     std::shared_ptr<ViewWrapper> focusedView() const override;
+    QVector<std::shared_ptr<Window>> windows() const override;
+    std::shared_ptr<Window> qobjectAsWindow(QObject *) const override;
+    virtual std::shared_ptr<Window> windowFromQWindow(QWindow *) const = 0;
 };
 
 }