Remove unneeded interal class TabbedViewContainerLayout

That internal class was used to workaround the problem that
QBoxLayout::insertItem() is a protected method. Since the current codo
does not use that method any more, no need to keep and use that internal
class anymore. Just use QVboxLayout directly.

src/ViewContainer.cpp

@@ -483,7 +483,7 @@ TabbedViewContainer::TabbedViewContainer(NavigationPosition position , QObject*
     _tabBarLayout->addWidget(_tabBar);
     _tabBarLayout->addWidget(_closeTabButton);
 
-    _layout = new TabbedViewContainerLayout;
+    _layout = new QVBoxLayout;
     _layout->setSpacing(0);
     _layout->setContentsMargins(0, 0, 0, 0);