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.