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