read back RDF annotations from semantic blackboard.

[kohlhase]

Once we have a connection to the semantic blackboard (SB) and can write annotations as RDF to it, we need to also be able to read back annotations from it, so that we can manage them (see #3). For that we need to also load the respective annotation specifications.

Eventually, we may want to also selectively only read partial annotations (e.g. given by a SPARQL query). But that is much further along.