Change Detection for (Entity, Trait) Pairs

[Ctrlmonster]

Continuing from #16:

Motivation for (Entity, Trait) Pairs is that the user can easily affect performance in a bad way should they add a change event to a trait that exists on many entities. E.g. let's say I have GUI component that reacts to changes in position on the player. Now I'm suddenly tracking changes on all entities with position even though I'm only interested in a single instance.

Also in my experience I'm seldom interested in changes on all instances of a trait. The Changed query modifier is the only case I can think of, but I haven't used it in practice either. Far more often I'm reacting to changes on a single entity (GUI being the most prominent).
That's a usecase a large percentage of applications has, that's why I think it's important we make sure it doesn't affect performance more than it needs to.

Challenges:
The imperative api's would need to be extended to support single entities to be passed. Tracking all traits should still be possible as that's important for the Changed modifier.