This will allow for more control over which ID should actually be marked
as unused/to be deleted.
Also some general minor cleanups and refactor, most notably:
* Add a constructor to internal `UnusedIDsData` struct, which can
directly use the public `LibQueryUnusedIDsData` parameters struct.
* Add default values to trivial data in `LibQueryUnusedIDsData`.
No expected behavioral changes in this commit.
Bastien Montagne
3fdae6e56d