This takes the `position` property and ensures children are sorted by
it, splits children by `pack-type` (also reversing the order of `end`
children), and handles children with `type="center"`.
If either a center child or end children exist, then the `GtkBox` is
converted to a `GtkCenterBox`, with `start-widget`/`end-widget` being a
nested `GtkBox` with the relevant children.
The splitting does cause some non-`object` children to sort differently
(hence the change to `office-runner.expected`.)
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