* Test entity labels, fixed corner cases, changed interface
* add tests for entity_labels_system
* fixed filling label_entities map
* fixed corner cases when removing entities, Labels component
* changed EntityLabels::get to return slice or empty slice instead of
None or Some empty or non-empty slice
Changing the interface of EntityLabels::get is beneficial, since else
you would get different results in case there was an entity before that
with this missing label or not. You would either get None or Some(&[])
and need to handle both, which is actually not necessary.
* register type Labels in CorePlugin
