The Purge operator to remove unused IDs can now also remove 'indirectly
unused' data-blocks (those only used by unused ones, recursively).
It can also now only operate on linked, or on local data.
All those options are exposed in the `File -> Cleanup` main menu.
The behavior of the `Purge` button in the Outliner remains unchanged,
needs some UI/UX design task for that.
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