Blender Git Loki

Move cache key struct (and its hashing and compare utils) to BKE_idtype.

Since those are generic to IDTypeInfo callbacks, move them here, they do
not belong to BLO anymore for sure.

Cleanup for previous commit.

Initial Refactor of code preserving caches during undo.

This is a proof od concept, it only implements new system for bSound
data-block as test subject.