Renamed class

Renamed UML2ExtlibPreparer from
UML2UMLWithPlaceholderActivitiesConverter
Modified Manifest; renamed UML2ExtlibPreparer (old), ...

Modified extlib Manifest to export itself as a package (to be used in
extlib.test); renamed to UML2Preparer from UML2ExtlibPreparer; provided
inital empty interface of IntegrationLayer that extends
ExecutionEventListener