This means once an ID is created,
it will keep using the same PyObject instance.
This has some advantages:
- Avoids unnecessary re-creation of instances on UI poll / redraw.
- Accessing free'd ID's gives an exception instead of crashing.
(long standing annoyance!, though this only applies to ID's
and not yet other data that uses the ID's - vertices for eg).
- Allows using instance comparison (a little faster).
Note that the instances won't be kept between undo.
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