Blender Git Loki

Undo: Initial wip useless to handle ID pointers issues through more mappings.

Don't think this is a working solution in fact, for at least two
reasons:
* Chaining Undo and Redo steps in random paterns with some ID being
reused and others being new all the time would require a very complex
system to keep track of all those changes and keep being able to remap
properly pointers from unread datablocks the the right new memory of
read datablocks.
* As small as it is, there is a risk of pointer collision here, and I
cannot see how to 100% handle it safely and sanely.