This is essentially a huge refactor/extension of our existing RNA
compare & copy code, since static override needs more advanced handling here.
Note that not all new features are implemented yet, advanced things like
collections insertion/deletion are still TODO (medium priority).
This completes the ground work for overrides, remaining commits will be
about UI and some basic/testing activation of overrides for a limited
set of data-blocks & properties.
For details see https://developer.blender.org/D2417
638afb9bd4