CorePlugin: Delete navigation(sub)widgets when deleting the mainwindow

Otherwise we might leak them on shutdown.

Change-Id: Id7274e6ee67b11fa9ff43b22fc6d5e62da55933f
Reviewed-by: hjk <hjk@theqtcompany.com>
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/coreplugin/mainwindow.cpp

@@ -297,6 +297,9 @@ MainWindow::~MainWindow()
     PluginManager::removeObject(m_outputView);
     delete m_outputView;
 
+    delete m_navigationWidget;
+    m_navigationWidget = 0;
+
     delete m_editorManager;
     m_editorManager = 0;
     delete m_progressManager;