Add a per-event entity cache

[GGAlanSmithee]

Currently, there are ongoing changes to greatly simplify the API of the EntityManager and other classes. This has been done without affecting performance etc too much, but there are some places where it will be slower, such as when iterating entites.

To further mitigate the negative impacts of theese changes, we should add a per-event entity cache, much like we have per system. This would cache all entites which have the components that the event was registered on.

This will actually make events even easier to work with, and there would still be the option to iterate/access all entities if needed.