Fix exit crash when having context help open.

Add a shutdown()-notification to MainWindow and
reset the context in it.

Task-number: 256386
Reviewed-by: con <qtc-committer@nokia.com>

src/plugins/coreplugin/coreplugin.cpp

@@ -96,4 +96,9 @@ void CorePlugin::remoteArgument(const QString& arg)
     }
 }
 
+void CorePlugin::shutdown()
+{
+    m_mainWindow->shutdown();
+}
+
 Q_EXPORT_PLUGIN(CorePlugin)