qtquick: Fix crash when dereferencing incomplete stack controler

The stack controller CTOR was still in the stack (pun intended), then
dereferenced by tabbar. Instead, call Stack_qtquick::init() to intitialize
the the TabBar. At this point the Stack controller is already usable.

src/qtquick/views/Stack_qtquick.cpp

@@ -26,7 +26,6 @@ Stack_qtquick::Stack_qtquick(Controllers::Stack *controller,
                              Controllers::Frame *parent)
     : View_qtquick(controller, Type::Stack, Views::asQQuickItem(parent))
     , m_dockWidgetModel(new DockWidgetModel(this))
-    , m_tabBar(new Controllers::TabBar(controller))
     , m_stack(controller)
 {
     connect(m_dockWidgetModel, &DockWidgetModel::countChanged, this,
@@ -40,6 +39,17 @@ Stack_qtquick::Stack_qtquick(Controllers::Stack *controller,
                 Q_EMIT countChanged(); });
 }
 
+void Stack_qtquick::init()
+{
+    m_tabBar = new Controllers::TabBar(m_stack);
+    Q_EMIT tabBarChanged();
+}
+
+Controllers::Stack *Stack_qtquick::stack() const
+{
+    return m_stack;
+}
+
 void Stack_qtquick::setDocumentMode(bool)
 {
     qDebug() << "Not implemented";