A few ID types are considered as 'never unused' in Blender (UI related
ones, the Libraries and the Scenes). Local IDs of this type are always
considered as used, even if no other ID links to them.
This was previously fairly weekly defined and implemented (mainly in the
writefile code and the 'tag unused' libquery functions).
This commit formalize this characteristic of ID types by adding a new
`IDTYPE_FLAGS_NEVER_UNUSED` flag, and using this in the few places in
the code that handle unused IDs.
6426de4489