deletingEntities is pretty unsafe

[jgrewe]

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.