Admin to set some portlets to out-of-service state at runtime.

The PortletTrackingManager renders "Portlet is not responding and has been taken out of service." when a portlet window is timed-out.
I think we can imporove this with the following features:

• Administrator can mark a portlet or a portlet application as "out-of-service" in a UI.
• PortletTrackingManager read the above information as well to decide if the portlet window is out-of-service.
• The message for the out-of-service portlets can be configured.