Decouple DockWidget from TabWidget

TabWidget will be an implementation detail of FrameWidget, and not
shared by QtQuick

src/private/widgets/FrameWidget_p.h

@@ -43,6 +43,9 @@ protected:
     void paintEvent(QPaintEvent *) override;
     QSize maxSizeHint() const override;
     void detachTab(DockWidgetBase *) override;
+    int indexOfDockWidget(DockWidgetBase *) override;
+    void setCurrentDockWidget(DockWidgetBase *) override;
+    int currentIndex() override;
 };