Blender Git Loki

Heavily rework Amber's data model.

Since we want to be able to edit repo from within Blender, we'll need
UI-compatible representation of the repo.

Alas, PropertyGroup py instances are regenerated all the time, which
means we cannot store py-only data in those, so we have to go to a
triple representation of repo here (the JSON/dict one, the effective py
classes, and the UI-compatible PropertyGroup-based one).

Note that this is rather heavily WIP commit, it most certainly breaks
several things, but at least the basic repo listing from filebrowser is
working again...