Use multiple entity label properties

For the first prototype, we used the entities, their abstracts, their types and the labels of the entities and the types for extracting sentences containing an entity and their type. But many entities have more than one label, e.g., the abstract of `http://dbpedia.org/page/Leonardo_da_Vinci` starts with:

```
Leonardo di ser Piero da Vinci (...) was an Italian Renaissance polymath ...
```

while its `rdfs:label` is `"Leonardo da Vinci"`.

We should use additional labels, e.g.,

```
dbpprop:alternativeNames "Leonardo di ser Piero da Vinci"
dbpprop:birthName "Leonardo di ser Piero da Vinci"
```