Force active ID cache regen on space change after adding windows

[ianyh]

Closes #674

[ianfixes]

You should add some comments to Change.swift; I had been under the impression that this was something to do with event management, not cache management. There is some mystery (to me) to where & how we have to do this sort of thing, and I was really hesitant to touch any of that code for that reason.

I guess what I'm saying is that I wonder if any code that I contributed may have benefited from understanding the ID cache.

[ianyh]

The change to that enum is unrelated. I was just adding more meaningful values than unknown because it was hard to debug otherwise.