qtquick: Implement View::windowHandle() and Window_qtquick::rootView()

For QtWidgets it now calls create() in the ctor so we have a window
during tests.

src/qtwidgets/TestHelpers_qtwidgets.cpp

@@ -26,11 +26,14 @@ namespace KDDockWidgets {
 class TestView_qtwidgets : public Views::View_qtwidgets<QWidget>
 {
 public:
-    using Views::View_qtwidgets<QWidget>::View_qtwidgets;
+    explicit TestView_qtwidgets(QWidget *parent)
+        : Views::View_qtwidgets<QWidget>(nullptr, Type::None, parent)
+    {
+        create();
+    }
 };
 }
 
-
 Platform_qtwidgets::Platform_qtwidgets(int argc, char *argv[])
     : Platform_qt(argc, argv)
 {
@@ -54,7 +57,7 @@ View *Platform_qtwidgets::tests_createView(View *parent)
 {
     QWidget *parentWidget = Views::View_qtwidgets<QWidget>::asQWidget(parent);
 
-    auto newWidget = new TestView_qtwidgets(nullptr, Type::None, parentWidget);
+    auto newWidget = new TestView_qtwidgets(parentWidget);
     return newWidget;
 }