The only real reason we need `butspacetype` is while switching areas, where we
need to delay the actual switch to the RNA _update callback since only there we
can access context.
So instead of trying to sync it with `spacetype`, only set while needed and
unset it afterwards (as in set to `SPACE_EMPTY`).
This should also allow us to re-use `butspacetype` in versioning code when
trying to read removed editors. It'll store the space type value of the removed
editor which we can then use on versioning.
For backwards compatibility, we store `butspacetype` with the value of
`spacetype`.
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