Use QActions to fix toolbar extension

Instead of directly adding QToolButtons to the tool bar of the
navigation subwidget, use QActions. This allows the tool bar extension
to show them in a menu when necessary.

The icons are inappropriate for in a menu, however.

src/plugins/coreplugin/navigationwidget.h

@@ -151,7 +151,7 @@ private:
     NavigationWidget *m_parentWidget;
     QComboBox *m_navigationComboBox;
     QWidget *m_navigationWidget;
-    QToolBar *m_toolbar;
+    QToolBar *m_toolBar;
     QAction *m_splitAction;
     QList<QToolButton *> m_additionalToolBarWidgets;
 };