PersistencyService: Add load listen method.

* The current implementation of the IPersistencyService does not
  allow to listen for "load" or "add" events during the startup of
  AF3 whereas later events for adding and removing IToplevelElements
  are present.
* Extend the PersistencyServiceListener and the IPersistencyService
  for such a capability to enable installing listeners for model
  elements in a plugin-in fashion.

Issue-Ref: 3514
Issue-Url: https://af3-developer.fortiss.org/issues/3514


Signed-off-by: Alexander Diewald <diewald@fortiss.org>