Steps to reproduce.

1. Load a plugin that adds a PluginManagerListener - note that the listener is called when all the plugins are loaded.

2. After all plugins are loaded, add a different plugin that also adds a PluginManagerListener after it has been initialised.

Expected results;

• The second plugin is notified that all plugins are loaded when it adds a PluginManagerListener 

Actual results:

• Both the first and second plugins are notified when the second plugin adds a PluginManagerListener.