[Bf-committers] Proxy Removal for Blender 3.0

[Bastien Montagne]

Hi fellow users & developers

Now that library overrides gained maturity and are production ready in 
the 'character animation' case (where they replace the old 'proxy' 
system), it is time to think about removing those proxies.

Plan is to commit in the coming days the following changes:
  - Add auto-conversion of proxies into library overrides on file load;
  - Remove the 'make proxy' operator.

For the time being, an option (OFF by default) will be added to the 
'Experimental' part of the User Preferences to skip converting existing 
proxies on file load. That way it is still possible to force keeping 
proxies if absolutely necessary, and/or for test purposes.

If everything goes well this option will be removed towards the end of 
BCon2 (end of October 2021). Full cleanup of internal proxy evaluation 
and management code can then happen during BCon1 of Blender 3.1.

Cheers,
Bastien