Blender Git Loki

LibOverrides: Refactor how diffing of RNA collections is handled.

Original implementation was a quick prototype which should have never
landed as-is in master. It had very limiting constraints and did not
allow for any real further development.

This commit fixes the internal implementation to make more sensible,
maintainable and evolutive.

NOTE: This commit introduces another forward-incompatibility in the
Blender file format: Files saved after this commit won't open properly
in older versions of blender regarding local inserted constraints or
modifiers into overrides of linked data.

NOTE: Technical details: The 'anchor' item name/index is now stored in
`subitem_reference_` members, and the actual 'source' item name/index is
stored in `subitem_local_` members of the override property operation
data.
Previously, only the `subitem_local_` members were used, storing the
anchor item name/index, and assuming the 'source' item was always the
next in the list.

Milestone I of T82160.

Maniphest Tasks: T82160

Differential Revision: https://developer.blender.org/D13282