Create ExternalComponent listener that will listen and may deny new settings

Create a new interface ExternalComponentManagerListener that will listen to events such as:

1. service enabled/disabled

2. port changed

3. default secret changed

4. policy changed

5. component allowed or blocked

6. changed component secret

7. delete component configuration

Change external-components-settings.jsp so that it can show an error message when a modification operation was not allowed by a listener.