Blender Git Loki

Fix (unreported) design flow in how workspace's relation data are read from .blend file.

Relying on pointer addresses across different data-blocks is extremely
not recommended (and should be strictly forbidden ideally), in
particular in direct_link step of blend file reading.
- It assumes a specific order in reading of data, which is not ensured
in future, and is in any case a very bad, non explicit, hidden
dependency on behaviors of other parts of the codebase.
- It is intrinsically unsafe (as in, it makes writing bad code and making
mistakes easy, see e.g. fix in rB84b3f6e049b35f9).
- It makes advanced handling of data-blocks harder (thinking about
partial undo code e.g., even though in this specific case it was not
an issue as we do not re-read neither windowmanagers nor worspaces
during undo).

New code uses windows' `winid` instead as 'anchor' to find again proper
workspace hook in windows at read time.

As a bonus, it will also cleanup the list of relations from any invalid
ones (afaict it was never done previously).

Differential Revision: https://developer.blender.org/D9073