Blender Git Loki

Cleanup: Move Outliner runtime hash into internal runtime struct, out of DNA

This way Outliner internal data stays internal, non-Outliner code will not be
able to access and mess with this. Further it allows us to use the real type
(rather than `void *`), change the type to a C++ container if needed and
slightly reduces the size for every Outliner stored in files.

Slightly changed how we set the `SO_TREESTORE_REBUILD` for this, but it should
effectively behave the same way as before.