Returns a unique identifier for the given part. The identifier can later
be used to restore the original part by calling getPart(...). This identifier
is guaranteed to be unique within a particular StackPresentation. However,
the same part may be assigned a different ID each time the presentation is saved.
Returns a presentable part, given an id that was generated when the presentation
id - an ID that was generated by getId(IPresentablePart) when the presentation
the presentable part associated with the given id, or null if none. Note
that even if the ID was valid when the presentation was saved, it may not
be valid when the presentation is restored. Callers must be prepared
to handle a null result.