Move TabWidget usage to FrameWidget

Frame is abstract and doesn't care about such implementation details.
QQuick will role out their own QTabWidget equivalent. No need to
abstract tab widget too, it's overkill.

src/private/widgets/FrameWidget.cpp

@@ -71,6 +71,11 @@ FrameWidget::FrameWidget(QWidget *parent, FrameOptions options)
     m_tabWidget->setTabBarAutoHide(!alwaysShowsTabs());
 }
 
+FrameWidget::~FrameWidget()
+{
+    m_inDtor = true;
+}
+
 void FrameWidget::paintEvent(QPaintEvent *)
 {
     if (!isFloating()) {