deletingEntities is pretty unsafe

Just stumbled over a strange behavior:

```
Section section = file.createSection("testSection");
file.deleteSection(section.id());
section.createProperty("testProperty"); //passes without complaint
```

Not sure, but what do you think about changing deleters to:

```
deleteEntity(Entity &entity);
```

this would allow the deleter to set the enity to none_t.