QMainWindow: allow positioning of tabbed dock widget's QTabBar

When tabbing QDockWidgets within QMainWindow using tabifyDockWidget(), a QTabBar is implicitly created at the bottom of the tab pages with a default shape of QTabBar::RoundedSouth.

Setting this QTabBar's shape property to a different QTabBar::Shape value does not honor the position that is desired.

Although the "shape" of the tabs will change, the position will always remain at the bottom of the pages.