- Using 'activeBits' field now to rebuild the entity list of each
EntitySystem when an entity is inserted/removed. It will be relatively
slow for systems that have many entities but only get those affected
every now and then. But the previous method was essentially doing the
same kind of data copying after a couple insertions/removal, worse if
there were more, and the binary search didn't helped either.
- Had to make EntityManager's entity list package private for easy
access.