Gltf-blender-io: Blender 2.8 conversion and integration

[UX3D-nopper]

• [x] Separate 2.79 and 2.80 code
• [x] Implement 2.80 code
• [x] Integrate into 2.80 repository

[julienduroure]

Some info about what will change (not final doc of course):
https://wiki.blender.org/wiki/Process/Addons/Guidelines/UpdatingScripts

[julienduroure]

Current status about timing (about 2.8)

• Beta release for Blender Conference (end of October). Probably that API will change until this beta release, but we can start working on it before, based on document previously posted.
• Final release start of 2019

[julienduroure]

Some remarks about 2.8:

• Not sure Blender Foundation will accept to have a single addon with both 2.79 & 2.80, like we have now, and may request a pure 2.8 code
• In 2.8, we have some errors about annotations now. We may have to duplicate our code for properties too