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/cppeditor/cppplugin.cpp

@@ -320,9 +320,10 @@ void CppPlugin::extensionsInitialized()
 {
 }
 
-void CppPlugin::aboutToShutdown()
+ExtensionSystem::IPlugin::ShutdownFlag CppPlugin::aboutToShutdown()
 {
     writeSettings();
+    return SynchronousShutdown;
 }
 
 void CppPlugin::switchDeclarationDefinition()