Initializes and later shuts down an INavigator instance.

Called early with a non-null host instance, at which point the PDF portfolio navigator can initialize itself and add event listeners. This is likely to occur shortly after construction and may occur before any children have been added (in the case of MXML components).

When the host shuts down, the PDF portfolio navigator calls this method once again with a value of null. At this point the INavigator instance is expected to shut itself down.