Handle entity insert/remove to systems when a component is added/removed at some point after creating the entity.

[unravel-dev]

Currently an entity is added/removed to/from a system only when it is created/destroyed. It should also be added/removed when a component matching the requirements is added/removed to the entity(maybe at a later point of the entity's existance).

[arvidsson]

Even though I don't recommend adding/removing components much inside the game loop, and it's not really how the design of this library was intended, it should be easy to add but probably optional to use as it would affect performance.