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.h

@@ -47,8 +47,9 @@ public:
     CorePlugin();
     ~CorePlugin();
 
-    bool initialize(const QStringList &arguments, QString *errorMessage = 0);
-    void extensionsInitialized();
+    virtual bool initialize(const QStringList &arguments, QString *errorMessage = 0);
+    virtual void extensionsInitialized();
+    virtual void shutdown();
 
 public slots:
     void remoteArgument(const QString&);