Blender Git Loki

Add optional, free-after-use usages mapping of IDs to Main.

The new MainIDRelations stores two mappings, one from ID users to ID
used, the other vice-versa.

That data is assumed to be short-living runtime, code creating it is
responsible to clear it asap. It will be much useful in places where we
handle relations between IDs for a lot of them at once.

Note: This commit is not fully functional, that is, the infamous, ugly,
PoS non-ID nodetrees will not be handled correctly when building relations.
Fix needed here is a bit noisy, so will be done in next own commit.