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

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

src/qtwidgets/Platform_qtwidgets.cpp

@@ -12,6 +12,7 @@
 #include "Platform_qtwidgets.h"
 #include "KDDockWidgets.h"
 
+#include "qtwidgets/Window_qtwidgets.h"
 #include "qtwidgets/DebugWindow_p.h"
 #include "qtwidgets/views/ViewWrapper_qtwidgets.h"
 
@@ -70,3 +71,8 @@ std::shared_ptr<ViewWrapper> Platform_qtwidgets::qobjectAsView(QObject *obj) con
 
     return nullptr;
 }
+
+std::shared_ptr<Window> Platform_qtwidgets::windowFromQWindow(QWindow *qwindow) const
+{
+    return std::shared_ptr<Window>(new Window_qtwidgets(qwindow));
+}