Consolidate toRemove, toEnable toDisable into toRefresh, move logic of
whether an entity should be added or removed to an EntitySystem into
EntitySystem.
Entities now have a systemBits member which represents which
systems they're currently a part of and is used by the refresh() logic
in EntitySystem.
Adding or removing components from an entity after it has been created
and activated will now add or remote the entity from relevant systems.
Matt Low
f366c4b808