Add functionality to iterate over all active entities

[Quillraven]

In my opinion this is a useful functionality for exceptional cases. Usually you have your systems to iterate over entities with a specific configuration but this might not be sufficient in all cases.

I could imagine a Save-/Loadsystem where you want to iterate over all entities at specific events like reaching a checkpoint.

I am not saying that this is the only solution to this problem but this functionality will allow the support of exceptional cases where systems might be not sufficient/cumbersome.