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/coreplugin/coreplugin.cpp

@@ -108,9 +108,10 @@ void CorePlugin::fileOpenRequest(const QString &f)
     remoteCommand(QStringList(), QStringList(f));
 }
 
-void CorePlugin::aboutToShutdown()
+ExtensionSystem::IPlugin::ShutdownFlag CorePlugin::aboutToShutdown()
 {
     m_mainWindow->aboutToShutdown();
+    return SynchronousShutdown;
 }
 
 Q_EXPORT_PLUGIN(CorePlugin)