Blender Git Loki

Implement T77959: Never duplicate linked data during deep-copy.

Note that this behavior is enforced on user level for now, but on code
side it is controlled with a flag, which should make it easy to refine
that behavior if needed.

Only exception is when we duplicate a linked ID directly (then we assume
user wants a local deep-copy of that linked data, and we always also
duplicate linked sub-data-blocks).

Note that this commit also slightly refactor the handling of actions of
animdata, by simplifying `BKE_animdata_copy_id_action()` and adding an
explicit new `BKE_animdata_duplicate_id_action()` to be used during ID
duplication (deep copy).

This also allows us to get rid of the special case for liboverrides.