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/libs/extensionsystem/pluginmanager.h

@@ -93,6 +93,7 @@ public:
     // Plugin operations
     QList<PluginSpec *> loadQueue();
     void loadPlugins();
+    void shutdown();
     QStringList pluginPaths() const;
     void setPluginPaths(const QStringList &paths);
     QList<PluginSpec *> plugins() const;