Blender Git Loki

LibOverride: Add concept of 'embedded'/'virtual' override.

IDs like embedded ones (master collections, root node trees) cannot be
linked, and thus cannot be real override themselves.

Since they are managed by their owner ID, that one will also have the
overrides for their locally edited properties.

We still need a way to mark them as overridden though, for various UI
and override-internal purposes, this is done using a new ID flag.

Note that since Shae Keys are not linkable, and their pointers are not
editable in RNA, they are also considered as embedded from override
point of view.