Extension branch for Blender 4.2

[vanous]

This is preparation for the Blender 4.2 extension support and store. The extension support looks pretty good. It allows using wheels, allowing us to move away from vendoring dependencies and having to modify them to use relative imports, then being stuck on old versions.

• Figure out distribution for 4.2
  • Extension store - yes
  • allow separate downloads as till now - yes
• Figure out backwards compatibility support for 4.
  • main for 4.1, extension for 4.2
  • once 4.2 is released, make main → legacy_addon and extension → main
• Convert everything to relative imports
• Convert imports to not use import *
• Migrate dependencies to wheels
• Migrate Preferences to new structure
• Respect new Internet Access Preferences settings in Blender
• Check and test carefully that everything works
  • sACN
  • OSC
  • Art-Net
  • GDTF
  • MVR
  • MVR-xchange