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/mainwindow.h

@@ -89,6 +89,7 @@ public:
 
     bool init(QString *errorMessage);
     void extensionsInitialized();
+    void shutdown();
 
     IContext *contextObject(QWidget *widget);
     void addContextObject(IContext *contex);