Add Frame::detachTab()

The tab widget will be an implementation detail of FrameWidget and
not accessed by anyone

src/private/widgets/FrameWidget_p.h

@@ -42,6 +42,7 @@ public:
 protected:
     void paintEvent(QPaintEvent *) override;
     QSize maxSizeHint() const override;
+    void detachTab(DockWidgetBase *) override;
 };