Fixes #70: PooledEntity: entity not fully reset

componentOperationHandler was not set to null when an entity was removed from an engine, which messes with component changes.

I'm not sure what the description in #70 means with the signals not being reset correctly; the test I added seems to confirm that everything works as expected.
We cannot rely very much on this test because if we add a new member it won't test it gets properly reset.