Add flag to IPlugin::aboutToShutdown that allows asynchronous shutdown.

If a plugin requests asyncronous shutdown, the shutdown sequence does
not continue to deleting the plugins before it has sent a
asynchronousShutdownFinished signal. During that time an event loop
is running.

src/plugins/cpptools/cpptoolsplugin.cpp

@@ -160,8 +160,9 @@ void CppToolsPlugin::extensionsInitialized()
     m_modelManager->setHeaderSuffixes(mimeType.suffixes());
 }
 
-void CppToolsPlugin::aboutToShutdown()
+ExtensionSystem::IPlugin::ShutdownFlag CppToolsPlugin::aboutToShutdown()
 {
+    return SynchronousShutdown;
 }
 
 void CppToolsPlugin::switchHeaderSource()

src/plugins/cpptools/cpptoolsplugin.h

@@ -65,7 +65,7 @@ public:
 
     bool initialize(const QStringList &arguments, QString *error_message);
     void extensionsInitialized();
-    void aboutToShutdown();
+    ShutdownFlag aboutToShutdown();
     CppModelManager *cppModelManager() { return m_modelManager; }
     QString correspondingHeaderOrSource(const QString &fileName) const;